PanelMenu Button for compatibility and for using Menus when needed

[neuromorph]

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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.

[raujonas]

Thanks for your work, I appreciate it. Just wanted to let you know that I will look at the changes eventually, I have not forgotten it. Thanks for your patience!

[neuromorph]

Thank you for the update. I have added another commit for the menu. So now, clicking on executor buttons opens a menu with one item in it for the Settings. More items can be added as needed. It also standardizes the button further using the addToStatusArea API of main, else input handling does not work well.

You can try it out as per your convenience, leaving it here for whenever you are ready.