Fix some tomboy warnings

Fix some tomboy warnings

There were some warnings because of the use of EggVirtualModifierType,
but the type can be changed to the GdkModifierType. A close examination
of the code shows that they are already equivalent, anyway.

Some of the code that was in the eggaccelerators.c has been moved into
GTK+. Some research shows that libegg was a staging library to test
experimental code before moving it into GTK+. This explains why lots of
the eggaccelerators.c code is duplicated exactly in GTK+.