Avoid Segfauls in lots of methods when called with nil instead of FXApp, FXComposite or FXWindow objects

Remove some comments, to only from C++ usable methods

Allow more consistent read/write access to image data, that is also
usable with derived classes like FXJPGImage.

Moreover much faster String based access to the pixel data is possible,
now.

FXImage#data and FXMemoryBuffer is now deprecated.

previous swig's #setData implementation wasn't really useable

Calling other classes while garbage collection is generally dangerous and useless.

This happens with FXFileSelector.

FXWindow destructor calls recalc() and changeFocus() of it's parent windows.
Since these methods are routed back to Ruby code, but calling Ruby code from
GC isn't a good idea, we mark the parent window as "in_gc", so that it will
ignore recalc() and changeFocus() calls completely.

The parent window should also be scheduled to be free'd. In the other case,
the child window would have been marked as used.