Refactor drag and drop behaviour of applets from panel to other containments

Implements the following:

• Add an option to the applets hover dialog that says "Move to different panel/desktop..."
• When you click that option, you'll be able to drag and drop the applet even outside the panel, to another panel
• The drag and drop will only drop the applet when you actually drop it, not when it gets far enough from the panel
• I'll make sure to add visual feedback that you are dragging something around with the mouse, compared to the status quo where the widget stays still in the initial panel