Support GPM (General Purpose Mouse)

[sinelaw]

https://wiki.archlinux.org/title/General_purpose_mouse

from https://news.ycombinator.com/item?id=46148520 :

OP, I'm playing around with it a bit, but notice it only supports mouse input when running in terminal emulator under X, and doesn't support mouse events from GPM (in the way that mcedit does, for example).