PanelMenu Button for compatibility and for using Menus when needed

[neuromorph]

Hello,

This PR replaces St.BoxLayout with a standard Gnome Top Panel PanelMenu.Button and its container Bin. This allows compatibility with other extensions like Open Bar and with custom user themes expecting standard buttons layout. Also, it takes steps towards building a Menu as discussed in this issue. I have currently created PanelMenu Button with a DummyMenu set to true (meaning no menu exists) which can be set to false when needed and Menu items can be added.

Cheers.