Calling any Ruby code within the mark phase is generelly dangerous,
because it usually allocates some objects.
Since most of the methods of FX objects can be overloaded in Ruby,
the calls in markfuncs now call explizitly the C++ methods.

Also remove some double marking of objects.

The target objects stored in the accelerator table are currently
private. Therefore FXRbAccelTable::markfunc() doesn't mark them as used.
As a workaround the objects are additionally stored in @targets Hash.

C++ source and header files.

This reverts commit 13dbbd0f.