Add comment to generate_symbol_map
Integrate startup and init_maincontext

A new function called initstart_maincontext is introduced
which executes startup_platform, init_maincontext,
and a user specified function.
After the user function returns free_maincontext
will be called and initstart_maincontext returns to the caller.
Rename .../shared/X11 into .../X11

KONFIG_html5 is removed -
I will concentrate my development effort on OpenGL instead.
KONFIG_opengl is introduced to be able to switch off OpenGL
support in an X11 environment.
Refactor buffer of x11window_t into x11dblbuffer_t

The double buffer support is moved from x11window_t
into x11dblbuffer_t.

Event handler functions are renamed from handle... into on...
(handledestroy_x11window into onDestroy_x11window ...).