Modified the procedure to get the widget at a point inside the pannable area when...

Modified the procedure to get the widget at a point inside the pannable area when using the movement type signals. Now the user has to call a function to get the child, that way we just calculate the child in case the user really want to use it and not all the time.

* src/hildon-marshalers.list: Modified the marshallers, now we do
	not have the widget as a parameter.

	* src/hildon-pannable-area.c
	(hildon_pannable_area_motion_notify_cb),
	(hildon_pannable_area_class_init),
	(hildon_pannable_get_child_widget_at):  Removed the widget
	parameter from the signal callback. We have also refactored the
	emission of the signals.

	* src/hildon-pannable-area.h: Added a new method to the API to
	calculate a widget at a point inside the pannable area,
	hildon_pannable_get_child_widget_at.

	* examples/hildon-pannable-area-example-4.c
	(horizontal_movement): Modified the example to use the new API,
	added condition to control which widget is under the event. This
	way we can avoid scrolling laterally over other widgets.