GDISP and GWIN button changes

GDISP:
Add polygon drawing
Add convex polygon filling
Start support for a monochrome display
Move gdisp doxygen to header file
Restructure the balance between gdisp.h and gdisp_lld.h
Remove old extern font definitions (now always use gdispOpenFont)
Make public the GDISP structure and change macros to suit (faster for the application).
gdispQuery is now optional.

GWIN buttons:
The user can now supply a custom button drawing routine
Arrow buttons are included as standard (using the new GDISP polygon support)