gtk-reftest: Allow loading code for reftests

gtk-reftest: Allow loading code for reftests

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.