Add room bookmarking: bookmarks are always on top

Introduce a bookmark system for rooms:
a room bookmarked will always appear on top of every other
(for a given account).

Bookmarking can be done through context menu on targeted room
(as well as un-bookmarking) or through settings.json
(key roomBookmarkIDs) for permanent bookmark.
roomBookmarkIds is a {account_user_id: [room_id]} dict.

src/gui/MainPane/RoomDelegate.qml

@@ -63,7 +63,9 @@ HTile {
                 spacing: room.spacing
 
                 TitleLabel {
-                    text: model.display_name || qsTr("Empty room")
+                    text:
+                        (model.bookmarked ? "\u2665 " : "") +
+                        (model.display_name || qsTr("Empty room"))
                     color:
                         model.local_unreads ?
                         theme.mainPane.listView.room.unreadName :
@@ -150,6 +152,14 @@ HTile {
             })
         }
 
+        HMenuItem {
+            icon.name: model.bookmarked ? "bookmark-remove": "bookmark-add"
+            text: model.bookmarked ? qsTr("Remove bookmark"): qsTr("Bookmark")
+            onTriggered: py.callClientCoro(
+                model.for_account, "toggle_bookmark", [model.id]
+            )
+        }
+
         HMenuItem {
             icon.name: "copy-room-id"
             text: qsTr("Copy room ID")