* gtk/gtktextlayout.c: Fix a warning.

	* gtk/gtktextsegment.h:
	* gtk/gtktextsegment.c:
	* gtk/gtktextchild.c:
	* gtk/gtktextbtree.c:
	* gtk/gtktextmark.c:
	* gtk/gtktexttypes.h: Make the segment class pointers const.

	* gtk/gtktextbufferserialize.c: Make parser struct const.

	* gtk/gtkuimanager.c: Make parser struct const.