* image.c, typedefs.h: Add ALTER_DESATURATE alteration type.
* img-view.c, layout_image.c, layout_util.c, menu.c: Allow to grayscale
- the display of current image with [Shift]+[G] kyboard shortcut and
- 'adjust' submenu item.
+ the display of current image with [Shift]+[G] keyboard shortcut and
+ 'adjust' submenu item. Also fixed bug of [Shift]+[R] shortcut from also
+ activating refresh [R].
* pixbuf_util.[ch] (pixbuf_desaturate_rect): Implement grayscale
function.