Made color enum comply with naming scheme and coding style

 - renamed SCF_* to SGUI_*
 - made define out of enum
Removed a few unneded things

 - rectangle line intersection test
   -> only supported vertical/horizontal lines anyway, use
      rectangle rectangle intersection instead
 - filesystem abstraction
   -> only used by the font loading function anyway
   -> added font load from memory function instead
 - sgui_screen forward declaration in predef header
   -> does not exist anymore
Primitively moved canvas string rendering to backend

 - draw_text_plain function in canvas uses draw string function
   in backend
 - simply copied implementation to backend
 - TODO: use backend font rendering functions (e.g. xrender for xlib)