This allows setting a customizable trigger which may be used to ungrab
the display. Currently, the default is ctrl+alt+g and the only thing that
will be released is the shortcut inhibition.
We want a bit more control over things than we can get with GtkPicture so
do the snapshot/measure internally. This allows us to know where the
picture is to be allocated so that we can translate pointer events to the
location within the console.
We don't need to do this manually now that we have GtkPicture working
correctly with content-fit. We still want to maintain our own widget to
manage things like grabs and all that though.
