Added more protection to the pannable area timeouts

Added more protection to the pannable area timeouts

* hildon/hildon-pannable-area.c,
(hildon_pannable_area_remove_timeouts): Adding this new function
with code that removes the timeouts of the widget.
(hildon_pannable_area_dispose),
(hildon_pannable_area_unrealize): Use the new remove function to
stop timeouts when unrealizing and disposing.
(hildon_pannable_area_scroll_to): Added a CLAMP to the position
calculation to avoid negative values in some cases.

Fixes: NB#135179 (Remove pannable timeouts in unrealize function)