Multi-head support

[mkoskar]

It would be nice if slop can dynamically change between following "modes" (I'm thinking of some keyboard binding, single for cycling or more specific ones):

• selecting of windows (basically as it is now, following pointer)
• selecting of xinerama/randr heads (active, following pointer)
• select whole root window
• custom region

[naelstrof]

https://github.com/naelstrof/maim/issues/103