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/UI.qml

@@ -16,8 +16,7 @@ Item {
 
 
     property bool accountsPresent:
-        (modelSources["Account"] || []).length > 0 ||
-        py.startupAnyAccountsSaved
+        ModelStore.get("accounts").count > 0 || py.startupAnyAccountsSaved
 
     readonly property alias shortcuts: shortcuts
     readonly property alias mainPane: mainPane