The GTK UI resources have to be different resources between major versions.
When kept in one directory with slightly different filenames they were
difficult to distinguish and resource definition was difficult to update.
The resources for both gtk versions were stored in both builds resulting in
unneccessary increased binary size.
The new stratagy lists the UI files once in a single gresources file and only
includes what is required. If a UI file is added it must be provided in both
gtk2 and 3 or a build failure will occour rather than a runtime error.
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