gtk3: add gtk_XXX prefix to GDK integration init functions

GitHub: close #127

Because Init_gdk_screen() is conflicted with the same name function in
Ruby/GDK3 on Mac OS X.

We don't extern internal Init_*() functions by G_GNUC_INTERNAL but it
is not worked on Mac OS X. Because clang is used instead of gcc on Mac
OS X.

Reported by Sergio Campama. Thanks!!!

Backtrace is provided by Watson. Thanks!!!