window-list: Add id property to buttons

A string ID that uniquely identifies a button will allow to
serialize/deserialize the positions in the next commit.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/338>

extensions/window-list/extension.js

@@ -575,6 +575,10 @@ class WindowButton extends BaseButton {
         this._updateStyle();
     }
 
+    get id() {
+        return `window:${this.metaWindow.get_id()}`;
+    }
+
     _createTitleActor() {
         return new WindowTitle(this.metaWindow);
     }
@@ -714,6 +718,10 @@ class AppButton extends BaseButton {
         this._updateStyle();
     }
 
+    get id() {
+        return `app:${this.app.get_id()}`;
+    }
+
     _windowEnteredOrLeftMonitor(metaDisplay, monitorIndex, metaWindow) {
         if (this._windowTracker.get_window_app(metaWindow) === this.app &&
             monitorIndex === this._monitorIndex) {