Handle NUL in text in places that g_utf8_strlen () was used

Handle NUL in text in places that g_utf8_strlen () was used

While pango-layout doesn't allow NUL in text, the lower level API should
handle it correctly.  We were using g_utf8_strlen() in a number of
places.  This is problematic since that function stops processing at
NUL even if length>1.  We now use an internal pango_utf8_strlen() instead.

Inspired by:
Bug 588678 – pango crash: install a new theme from gnome-appearance-properties