Rework roommanager for improved stability

Fixes #645

- Active space handling is moved from QML to RoomManager
- Active tab in SpaceDrawer (space / no space / DM) is unified in a single variable
- RoomList & RoomPage loading is simplified: We're always pushing a RoomPage now; if there is no room, a placeholder is shown
- SpaceHomePage is moved into RoomPage; This replaces the entire push/replace room/spacehome logic
- If the current room is a space, the space home is shown, otherwise the timeline
- The concept of "previous room" is removed entirely. If we're leaving the active room, the placeholder room page is shown
- When clicking on a space in the list, the space room list is switched and the space home page is shown

In short, these changes should (after some initial regressions) lead to a less crashy NeoChat :)

src/qml/TimelineView.qml

@@ -422,4 +422,8 @@ QQC2.ScrollView {
     function positionViewAtBeginning() {
         messageListView.positionViewAtBeginning();
     }
+
+    function goToEvent(eventId) {
+        messageListView.goToEvent(eventId);
+    }
 }