GResource Icons

Stop using inline pixbufs and instead use gresource

As Gtk doesn't have native support for GResource in Gtk2 some utilities are needed

Also simplifies object icon loading as there is now a single way to get a types icon (dia_object_type_get_icon) rather than multiple similar-but-not-idential implementations

I intend to further simplify icon handling in Dia 2 but for now backwards compatability with inline icons is mantained to avoid breaking existing 3rd party objects

Unfortunately there is quite a bit of noise as vscode tries to normalise whitespace according to editor config

I'll leave this open a few days for comments