Move the private get_cursor_color method belongs to StyleContext. Change
the api so that retrieving both primary and secondary color is possible.
I left the method private for now, though it should probably be public
as all the other getters.
The gtkprivate.h header contains GtkWidget-specific private symbols that
are not useful except in a handful of cases. Basically everything
includes gtkprivate.h for the GTK_PARAM_* macros.
https://bugzilla.gnome.org/show_bug.cgi?id=632539