We currently allow infinite number of screenshot requests to be active at
the same time, which can "dos" the system and cause OOM.
So fail subsequent requests for the same sender when a screenshot operation
is already running.
https://bugzilla.gnome.org/show_bug.cgi?id=737456
Since we also support passing a basename now, clients might be
interested in knowing the path used to save the file.
Add an out argument to the interface for that.
https://bugzilla.gnome.org/show_bug.cgi?id=688004
