gr: Fix: Minimal UTF-8 support.

gr.char(u"test") failed because of unicode type.
Ensure that all strings are of type str before creating c_char_p.

Change-Id: If973c2706be7384cf6949cfb9b1d058a46ab513e
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/128
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>
Fixed a text rendering bug in GKSTerm

Change-Id: Iaefffed7f0a76eabd029c1493f8379de0c4480c2
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/129
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>