Provide an option to disable Client Side Decoration (CSD) for GitHub Desktop window

[alitajelsir]

Describe the feature or problem you’d like to solve

There is no way to disable CSD for GitHub desktop window (it doesn't work with gtk3-nocsd). The app can't use the system title bar.

Proposed solution

Provide an option to disable CSD for GitHub desktop window. This will provide uniform look for apps in KDE.

Additional context

Chromium with no CSD vs GitHub Desktop:

[shiftkey]

I think this might be related to #149

[alitajelsir]

Thanks I'll continue posting in #149 and close this issue. Thanks for your efforts.