Linux gsettings sync support

[knackstedt]

Is your feature request related to a problem? Please describe. I want xpra applications to inherit the gnome theme when they are launched.

Describe the solution you'd like Xpra syncs the GTK theme with the user running the xpra process.

Describe alternatives you've considered It might be possible to use some CLI argument to achieve this, though I don't know what that might be.

Additional context I can help test/debug if necessary.