plug-ins: add SCREENSHOT_CAN_DELAY_WINDOW_SHOT capability to screenshot.
This indicates if a delay can be inserted in-between the window pick and the actual shot. This is indeed not always possible, for instance using KWin API. Obviously this feature is only meaningful if SCREENSHOT_CAN_PICK_WINDOW feature is enabled as well. For instance X11 screenshot has the feature, but GNOME won't need it since there is no window selection (it simply snaps whatever is the active window).