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/Popups/ForgetRoomPopup.qml

@@ -21,7 +21,9 @@ BoxPopup {
             window.uiState.pageProperties.userId === userId &&
             window.uiState.pageProperties.roomId === roomId)
         {
-            pageLoader.showPage("Default")
+            window.mainUI.pageLoader.showPrevious() ||
+            window.mainUI.pageLoader.showPage("Default")
+
             Qt.callLater(popup.destroy)
         }
     })

src/gui/Popups/SignOutPopup.qml

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 
 import QtQuick 2.12
+import ".."
 
 BoxPopup {
     id: popup
@@ -28,7 +29,7 @@ BoxPopup {
         ok: button => {
             utils.makeObject(
                 "Dialogs/ExportKeys.qml",
-                mainUI,
+                window.mainUI,
                 { userId },
                 obj => {
                     button.loading = Qt.binding(() => obj.exporting)
@@ -44,10 +45,9 @@ BoxPopup {
             okClicked = true
             popup.ok()
 
-            if ((modelSources["Account"] || []).length < 2) {
-                pageLoader.showPage("AddAccount/AddAccount")
-            } else if (window.uiState.pageProperties.userId === userId) {
-                pageLoader.showPage("Default")
+            if (ModelStore.get("accounts").count < 2 ||
+                    window.uiState.pageProperties.userId === userId) {
+                window.mainUI.pageLoader.showPage("AddAccount/AddAccount")
             }
 
             py.callCoro("logout_client", [userId])