app: pass the startup monitor to displays opened from the copmmand line

Change gimp_get_display_name() to also return the screen, and its
implementation in the GUI to return the initial monitor during
startup. Retrieve that information in app.c using a weird callback
construct and pass the monitor to file_open_from_command_line().

Half-related, add screen and monitor parameters to GimpDisplayShell
and use these initial values for calculating the canvas's initial
extents.

The image windows still don't position themselves correctly though
because we have no mechanism for that whatsoever just yet, but we now
at least pass the needed monitor information to the right objects.