[applet] Make the "force mute" plasmoid action checkable

applet/contents/ui/main.qml

@@ -706,5 +706,7 @@ Item {
         MicrophoneIndicator.init();
 
         plasmoid.setAction("forceMute", i18n("Force mute all playback devices"), "audio-volume-muted");
+        plasmoid.action("forceMute").checkable = true;
+        plasmoid.action("forceMute").checked = Qt.binding(() => {return globalMute;});
     }
 }