lib: text: Assert that style->font is not NULL.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
pywrap: text: Add wrappers for GP_TextStyle.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: py_simple: Add font style example.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>