0032308: Configuration - make Xlib dependency optional

0032308: Configuration - make Xlib dependency optional

MACOSX_USE_GLX has been renamed to HAVE_XLIB and now configurable on Linux target.

Xw_Window.hxx now export class on non-Linux platforms.
OpenGl_Window now creates an off-screen EGL surface also in case of
window-less desktop setup (e.g. with disabled Xlib on Linux).

Draw_Window - code has been cleaned up.
Class definition has been unified across platforms when possible.
Unusued constructors have been removed.
Internal header files (MainWindow.h, init.h, etc.) have been renamed to .pxx
to avoid their unexpected distribution in "inc".