Add 'max' actions for screen corners

[Taylor-Finch]

Presently JWM has shortcut actions for maximizing a window to four sides of the screen but not for the corners, adding these will provide additional flexibility to the user when laying out a workspace.

Commit: https://github.com/Taylor-Finch/jwm/commit/8cf7e5070c5c7384d188304571940a5e5ec0a603