Bug 770424 - Themes: insensitive menu item are not readable.

Not sure if g_type_class_ref() can actually return NULL here, but let's
add a check, just in case.
Also unref() after since we ref-ed it ourselves.
Finally reorganize a bit to keep the private functions together and
named appropriately, clean some tabs and add a comment to remind of
further check/cleanup once we port to GTK+3.