Auto-generate constants using gobject-introspection
replace some of the manual enums
programatically access enums by symbol
move generation code to Gtk.jl + strip of _MASK
replace gdk masks
consts for gtk2 and gtk3 separately
remove redundant strings
merge, but also add Clang implementation of enum generator
cleanup some Gtk constants definitions
update gi_gen_consts
gconsts using GI
add some #define constants to Gtk generator script