[desktop] Draggable window

Summary: After hiding the titlebar users can't drag the window using it. The standard way to do it is to add a -webkit-app-region: drag property to our custom titlebar.

Test Plan:

• Check that you can drag the window using the titlebar
• Check that you can't drag using buttons

Reviewers: tomek, atul, abosh, kamil

Reviewed By: tomek, atul

Subscribers: kamil, ashoat

Differential Revision: https://phab.comm.dev/D5632