mate-screenshot: Fix area selection timing with keyboard shortcuts

[vkareh]

This prevents possible race conditions when launched via keyboard shortcuts. The delay allows keyboard handlers to release before input grabbing begins.

[cwendling]

As the hack already exists why not, but I'd rather see that on the other side of things, that is on the keybinding handlers or simply the commands themselves (I personally used something like sh -c 'sleep 0.25 && mate-screenshot --area'). The advantage of having this in the keybinding handler side is that it can affect any command, not just screenshot, plus on that end it could be possible to actually know when the keys were un-grabbed.

[vkareh]

@cwendling good point, but fixing the at the keybinding handler level seems like would bring up some edge cases I'm not sure I want to test :P

For now, I changed this commit to be more specifically targetted to area screenshots, and leave the else { g_idle_add } in place.