Implement close_windows/0 and keep_windows/0

[Seeker04]

menu:close_windows/0 should close all selected windows. menu:keep_windows/0 should do the complement: close all the non-selected windows.

Notes:

• dmenu can multi-select with the Ctrl+Enter key combination by default
• rofi can do the same with Shift+Enter when run using -dmenu -multi-select