Cache the HildonPannableArea panning indicator color

Cache the HildonPannableArea panning indicator color

* hildon/hildon-pannable-area.c: (hildon_pannable_area_init):
Cache the panning indicator color.,
(hildon_pannable_area_style_set): Query the color on style changes.
(hildon_pannable_draw_vscroll),
(hildon_pannable_draw_hscroll),
(hildon_pannable_area_expose_event): Use the cached panning indicator
color instead of looking it up on every expose and more.