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

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

This could hide namespace issues in fxruby.