Refactor SidePane ListView in a flat way

Also adjust some colors (accentBackground)

TODO: Repair Filter rooms field

src/qml/Window.qml

@@ -15,6 +15,7 @@ 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 sidePaneModelSource: []
 
     property var mainUI: null