Add screenshot interface

Adds methods to shell_global to allow taking screenshots
save the result into a specified png image.

It exposes three methods via shellDBus applications like
gnome-screenshot:

*) Screenshot (screenshots the whole screen)
*) ScreenshotWindow (screenshots the focused window)
*) ScreenshotArea (screenshots a specific area)

https://bugzilla.gnome.org/show_bug.cgi?id=652952