Add test cases for non Integer assignment to FXuchar, FXshort and FXint.

It's typically possible to assign Float values to methods expecting
Integers and we also shouldn't break compatibility.

This reverts commit 3360c669.

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.