Allow to set mouse buttons/keys as window modfier

Submitted by HJ

Assigned to Olivier Fourdan @olivier

Link to original bug (#11720)

Description

Hi,

I guess this would be a nice enhancement since it would allow us to fully control the windows with my mice eg: resizing/moving windows also many window decorations don't have huge borders which makes it nearly impossible to rezize them properly.

GTK3 headerbar windows have also no decorations or resize grips and using the window modfier key is often the only way to resize them.

Version: 4.12.0