Working on mention notification.

Currently works great. Only problem: does not respect the taskbar.

CANNOT find a fucking way to either
  (a) get the location of a non-Tcl window or
  (b) get the "total usable" space

For b, I tried maybe showing an invisible window maximized, getting
its geometry and figuring it out from there, but apparently on
X11 you can't fucking make a window Maximized.

https://www.ruby-forum.com/topic/203366
https://groups.google.com/forum/#!topic/comp.lang.tcl/7hZcaKBR-Is