* app/channel.[ch]
	* app/channel_cmds.c
	* app/drawable.h
	* app/channels_dialog.c
	* app/gimage_cmds.c
	* app/gimpdrawable.[ch]
	* app/gimpimage.c
	* app/layer.[ch]
	* app/layer_cmds.c
	* app/layer_select.c
	* app/layers_dialog.c: enforce unique names for layers and
	channels.  New functions {channel,gimp_drawable,layer}_{get,set}_name