GTK UI builder resources have till now been exclusively stored on disc
requiring netsurf to ship numerous additional resource files. This
requires going to disc every time a UI action is performed which can
become a lot of unwanted file handling.
GLib/GTK has moved towards GResource handling for such resources
instead. It now seems that migrating to this style of usage is
expected and indeed the only portable way to include pixbufs.
This introduces an API to hide the various implementation details of
how resources are handled from the rest of the codebase.
