Updated usage part of existing documentation

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Several bug fixes

 - utf8 strncpy compared unset destination value instead of source
 - edit box destroy didn't free shadow buffer
 - demo tried to wait for thread even if started without OpenGL demo
 - some not yet identified code appears to rely on previous behaviour
   of sgui_rect_get_intersection. Reverted to previous behaviour

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Moved tab order lookup function to widget

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Made event queue dynamically allocated and resized

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added dummy implementation for memory canvas creation

When memory canvas is turned off, the creation function is now still
implemented as a dummy function that always fails, to avoid
unexpected linking errors.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>