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/Pages/Chat/Timeline/EventContent.qml

@@ -172,7 +172,7 @@ HRowLayout {
 
         HRepeater {
             id: linksRepeater
-            model: eventDelegate.currentModel.links
+            model: JSON.parse(eventDelegate.currentModel.links)
 
             EventMediaLoader {
                 singleMediaInfo: eventDelegate.currentModel