GTK3: Switch to using GResource for compiled in image resources. Second commit with similar changes coming for GTK2 soon. dwindows-config --gresource will return "enabled" if GResource is supported. GLib 2.32 is required for GResource support, if GLib is older than 2.32 Dynamic Windows will fall back to the old style compiled in resource array.

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=3
DW_MINOR_VERSION=1