js: Always use SOURCE_CONTINUE/SOURCE_REMOVE in source functions

The constants are more self-explanatory (and therefore readable)
than plain true/false.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/435>

extensions/places-menu/placeDisplay.js

@@ -267,7 +267,7 @@ export class PlacesManager extends EventEmitter {
                     GLib.PRIORITY_DEFAULT, 100, () => {
                         this._bookmarkTimeoutId = 0;
                         this._reloadBookmarks();
-                        return false;
+                        return GLib.SOURCE_REMOVE;
                     });
             });
 

extensions/window-list/extension.js

@@ -1476,7 +1476,7 @@ class WindowList extends St.Widget {
         this._dndWindow = null;
         this._dndTimeoutId = 0;
 
-        return false;
+        return GLib.SOURCE_REMOVE;
     }
 
     _onDestroy() {