This adds an inhibit api that code from the reftest module
can use to delay the taking of the snapshot. Also refactor
the code in gtk-reftest to use the inhibit mechanism for
its own delaying of the snapshot until after the first
expose.
When connecting signal names, gtk-reftest now allows you to use a colon
in the signal handler name like so:
module:function_name
where module is a module loaded from the same directory (or the .libs
subdirectory for compatibility with uninstalled libtool) as the running
test and the function is resolved in that module. Of course, normal
function names work as before.
