Show spaces horizontal bar

### Summary

This merge request adds a horizontal bar at top of room list, which shows spaces. By clicking on a space, user can filter out rooms belonging only to that specific space.

### Pending/ Help needed

#### Segfault when loading active connection on startup

Refer `void SortFilterRoomListModel::cacheSpaceHierarchy()` ([link](8c372800d7 (b969e462c30df43ef3714ea441948d8d8027f6a0_117_126))) in `src/sortfilterroomlistmodel.cpp`.

On [line 129](8c372800d7 (b969e462c30df43ef3714ea441948d8d8027f6a0_117_129)), I have called `connection->allRooms()`, which segfaults if the active connection hasn't been loaded yet. Is there a way to ensure that `Controller::instance().activeConnection()` on line 128 waits till connection is loaded?

#### Avatars

Avatars on space horizontal bar aren't aligned to vertical middle. I'll need help with that. 

Using the code below doesn't help with padding

```qml
delegate: QQC2.Control {
    topPadding: 10
    contentItem: Kirigami.Avatar { ..... }
}
```

This complains about uninitialized properties in `Kirigami.Avatar`. (`id`, `currentRoom`, `avatar`, `index` are properties utilized during run time)

After we get around these two issue, this MR will be ready from my side.

imports/NeoChat/Page/RoomListPage.qml

@@ -15,6 +15,95 @@ import NeoChat.Component 1.0
 import NeoChat.Menu 1.0
 
 Kirigami.ScrollablePage {
+
+    header: ColumnLayout {
+        visible: !page.collapsedMode
+        Layout.fillWidth: true
+        Layout.fillHeight: true
+        spacing: 0
+
+        ListView {
+            id: spaceList
+            property string activeSpaceId: ''
+
+            orientation: Qt.Horizontal
+            spacing: Kirigami.Units.largeSpacing
+            clip:true
+            visible: spaceList.count > 0
+
+            Layout.preferredHeight: Kirigami.Units.gridUnit * 3
+            Layout.fillWidth: true
+
+            model: SortFilterSpaceListModel {
+                id: sortFilterSpaceListModel
+                sourceModel: RoomListModel {
+                    id: spaceListModel
+                    connection: Controller.activeConnection
+                }
+            }
+
+
+            Connections {
+                target: SpaceHierarchyCache
+                function onSpaceHierarchyChanged() {
+                    if (spaceList.activeSpaceId !== '') {
+                        sortFilterRoomListModel.activeSpaceRooms = SpaceHierarchyCache.getRoomListForSpace(spaceList.activeSpaceId, false);
+                    }
+                }
+            }
+
+            header: QQC2.Control {
+                contentItem: QQC2.RoundButton {
+                    id: homeButton
+                    flat: true
+                    padding: Kirigami.Units.gridUnit / 2
+                    icon.name: "home"
+                    text: i18nc('@action:button', 'Show All Rooms')
+                    display: QQC2.AbstractButton.IconOnly
+
+                    onClicked: {
+                        sortFilterRoomListModel.activeSpaceRooms = [];
+                        spaceList.activeSpaceId = '';
+                        listView.positionViewAtIndex(0, ListView.Beginning);
+                    }
+
+                    QQC2.ToolTip {
+                        text: homeButton.text
+                    }
+                }
+            }
+
+            delegate: QQC2.Control {
+                required property string avatar
+                required property var currentRoom
+                required property int index
+                required property string id
+                implicitWidth: ListView.view.headerItem.implicitWidth
+                implicitHeight: ListView.view.headerItem.implicitHeight
+
+                contentItem: Kirigami.Avatar {
+                    id: del
+
+                    actions.main: Kirigami.Action {
+                        id: enterSpaceAction
+                        onTriggered: {
+                            spaceList.activeSpaceId = id;
+                            sortFilterRoomListModel.activeSpaceRooms = SpaceHierarchyCache.getRoomListForSpace(id, true);
+                        }
+                    }
+
+                    QQC2.ToolTip {
+                        text: currentRoom.displayName
+                    }
+
+                    source: avatar !== "" ? "image://mxc/" + avatar : ""
+                }
+            }
+        }
+        Kirigami.Separator {
+            Layout.fillWidth: true
+        }
+    }
     id: page
 
     title: i18n("Rooms")
@@ -68,7 +157,6 @@ Kirigami.ScrollablePage {
         }
     }
 
-
     ListView {
         id: listView
 
@@ -100,6 +188,8 @@ Kirigami.ScrollablePage {
             }
         }
 
+        Layout.fillWidth: true
+
         Kirigami.PlaceholderMessage {
             anchors.centerIn: parent
             width: parent.width - (Kirigami.Units.largeSpacing * 4)