HildonPannableArea: new 'panning-started' and 'panning-finished' signals

HildonPannableArea: new 'panning-started' and 'panning-finished' signals

* hildon/hildon-pannable-area.c (hildon_pannable_area_class_init)
(hildon_pannable_area_dispose)
(hildon_pannable_area_button_press_cb)
(hildon_pannable_area_timeout, hildon_pannable_area_check_move)
(hildon_pannable_area_button_release_cb)
(hildon_pannable_area_scroll_cb, hildon_pannable_area_jump_to):
New 'panning-started' and 'panning-finished' signals.

Fixes: NB#121732 (New feature request: Signal to pannable area
that informs when panning is stopped.)