changed ModifierType to bitflags, more rust-like, and added a usage example to examples/gtktest
fix Modifiermask value

I believe it should be this value..
https://fossies.org/dox/gtkmm-3.14.0/types_8h_source.html
convert modifierintent to bitflags
make modifiertype used by GDk structs
Merge pull request #160 from buster/modifier-enums-reform

changed ModifierType to bitflags, more rust-like, and added a usage exam...