Check QAction enablement before triggering

QAction::trigger does not check enablement. File manager's undo action
gets enabled/disabled depending on whether we have anything to undo.

If we unconditionally invoke the method we can crash.

As an aside, the comment about QQuickItem and shortcut looks very
outdated.

src/scriptengines/qml/plasmoid/appletinterface.cpp

@@ -797,6 +797,10 @@ bool AppletInterface::event(QEvent *event)
                 continue;
             }
 
+            if (!a->isEnabled()) {
+                continue;
+            }
+
             // this will happen on a normal, non emacs shortcut
             if (seq.matches(a->shortcut()) == QKeySequence::ExactMatch) {
                 event->accept();