Use new PiP icons when available

Use the new "pip-in" and "pip-out" GNOME symbolic icons if user theme has them for enter/leave floating mode button. When icon is unavailable on the host, old icon will be used instead.

src/headerbar.js

@@ -9,17 +9,20 @@ class ClapperHeaderBar extends HeaderBarBase
         this.activate_action(`window.${action}`, null);
     }
 
-    _onFloatButtonClicked()
+    _onFloatButtonClicked(button)
     {
         const clapperWidget = this.root.child;
+        const { controlsRevealer } = clapperWidget;
 
-        clapperWidget.controlsRevealer.toggleReveal();
+        controlsRevealer.toggleReveal();
 
         /* Reset timer to not disappear during click */
         clapperWidget._setHideControlsTimeout();
+
+        this._updateFloatIcon(!controlsRevealer.reveal_child);
     }
 
-    _onFullscreenButtonClicked()
+    _onFullscreenButtonClicked(button)
     {
         this.root.fullscreen();
     }