Auto-generate all enums in Gtk/Gdk

This enables auto-generation of the complete set of consts and enums in Gtk/Gdk (using gobject-introspection). The support code is in https://github.com/bfredl/GI.jl . However there are a few outstanding issues:

Gobject-introspection does work on windows (used for instance by pygtk as of gtk3) but the introspection data is not currently included in the WinRPM packages. I think this comes down to enable some configuration flags in the WinRPM build scripts; I will look into it later. As with cindex it is only a dependency for regenerating the cache, not for using the package.
As of #24 "_MASK" are stripped of the Gdk mask enum values. If we want to keep this convention this could be done systematically also in the geneated enums, since libgirepo contains info which enums are in fact masks. Right now the enums are in a submodule but when this is resolved they could all be put in the main Gtk namespace.
This PR replaces the `EnumName.get` symbol lookup functions with  `getfield`, which probably is somewhat slower (cannot be constant folded I guess). If this is a problem `EnumName.get` could be generated as well (for all enums or a relevant subset of the enums).

Would people consider this useful? If so I will try fix the remaining issues, as needed.