Leave room support, persist last room, fixes

lib/widgets/chat_page/room_menu.dart

@@ -20,8 +20,32 @@ class RoomMenu extends StatelessWidget {
           child: ListTile(leading: Icon(Icons.link), title: Text("Copy Link")),
         ),
         PopupMenuItem(
-          onTap: () =>
-              showDialog(context: context, builder: (context) => AlertDialog()),
+          onTap: () => showDialog(
+            context: context,
+            builder: (context) => AlertDialog(
+              title: Text("Leave Room"),
+              content: Text(
+                "Are you sure you want to leave \"${room.getLocalizedDisplayname()}\"?",
+              ),
+              actions: [
+                TextButton(
+                  onPressed: Navigator.of(context).pop,
+                  child: Text("Cancel"),
+                ),
+                TextButton(
+                  onPressed: () async {
+                    Navigator.of(context).pop();
+                    final snackbar = ScaffoldMessenger.of(
+                      context,
+                    ).showSnackBar(SnackBar(content: Text("Leaving room...")));
+                    await room.leave();
+                    snackbar.close();
+                  },
+                  child: Text("Leave"),
+                ),
+              ],
+            ),
+          ),
           child: ListTile(
             leading: Icon(Icons.logout, color: danger),
             title: Text("Leave", style: TextStyle(color: danger)),