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/FileTransfer/TransferList.qml

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 
 import QtQuick 2.12
+import "../../.."
 import "../../../Base"
 
 Rectangle {
@@ -25,11 +26,7 @@ Rectangle {
         id: transferList
         anchors.fill: parent
 
-        model: HListModel {
-            keyField: "uuid"
-            source: modelSources[["Upload", chat.roomId]] || []
-        }
-
+        model: ModelStore.get(chat.roomId, "uploads")
         delegate: Transfer { width: transferList.width }
     }
 }