Add option to enable/disable frameless mode

@antiphon it’s easier for us to track things if we consolidate things (e.g. in this thread). I guess it was moved to the graveyard because the request is being tracked here, and because it technically is not a bug: Frameless mode was introduced somewhere in v0.9.11? and it works as intended. Adding an option to disable it is a new feature.

I understand this might be annoying for you, but Linux is not the only distro the app is designed for, so have a little patience! For other (perhaps more popular) OSs the frameless mode works better.

The only place to track progress is mentioned in the FAQ: Is there a roadmap?
Quoting the disclaimer from Discord:

Note: it only contains the bigger projects and improvements. Smaller things and bug fixes are not on there, as an attempt to reduce clutter.