Begin yet another model refactor

Use native ListModel which require a lot of changes, but should be
much faster than the old way which exponentially slowed down to a crawl.
Also fix some popup bugs (leave/forget).

Not working yet: side pane keyboard controls, proper highlight,
room & member filtering, local echo replacement

src/gui/Window.qml

@@ -28,7 +28,6 @@ ApplicationWindow {
 
     // NOTE: For JS object variables, the corresponding method to notify
     // key/value changes must be called manually, e.g. settingsChanged().
-    property var modelSources: ({})
     property var mainPaneModelSource: []
 
     property var mainUI: null
@@ -46,8 +45,6 @@ ApplicationWindow {
 
     property var hideErrorTypes: new Set()
 
-    readonly property alias py: py
-
 
     function saveState(obj) {
         if (! obj.saveName || ! obj.saveProperties ||
@@ -75,7 +72,7 @@ ApplicationWindow {
     }
 
 
-    PythonBridge { id: py }
+    PythonRootBridge { id: py }
 
     Utils { id: utils }