Improve the "Create a Room/Space" and "Select Existing Room" dialog

First of all, these are now all separate dialogs instead of shoving all
of these functions (which are only marginally related) into one single
dialog. We also convert these to in-app Kirigami Dialogs, which look a
bit nicer. I also touched up the UX in some places, such as adding
descriptions which were previously available to translators. I also hid
some not oft used options like setting a topic, which almost nobody does
before creating a room/space.

src/rooms/ExploreComponentMobile.qml

@@ -159,13 +159,9 @@ Kirigami.NavigationTabBar {
                         text: i18n("Create a Space")
                         icon.name: "list-add"
                         onTriggered: {
-                            pageStack.pushDialogLayer(Qt.createComponent('org.kde.neochat', 'CreateRoomDialog'), {
-                                connection: root.connection,
-                                isSpace: true,
-                                title: i18nc("@title", "Create a Space")
-                            }, {
-                                title: i18nc("@title", "Create a Space")
-                            });
+                            Qt.createComponent('org.kde.neochat', 'CreateSpaceDialog').createObject(root, {
+                                connection: root.connection
+                            }).open();
                             explorePopup.close();
                         }
                     }