Desktop keys (Update src/desktop/desktop-window.c)

Arrow keys logic for desktop and Enter key action binding. This is related to issue #246.

- Selection change with Arrow keys movement.
- Hold CTRL and move wiht Arrow keys, select/unselect with Space.
- Enter key opens the focused item.

I don't know if this is useful for you IgnorantGuru, now I selected the "next" branch, I think you guys are doing it this way?

Anyway, keep up the good job :)