Support FLTK 1.4

[pdhahn]

Please add support to VL for FLTK -- preferably, for the latest 1.4 version. Please also add corresponding example/test files.

Subclassing the FLTK Fl_GL_Window class, including translation between FLTK <-> VL events and integration with the FLTK run/execute framework, is probably the basis for this work.

Thanks for VL!