Added a new feature to the HildonPannableArea allowing center on the child

Added a new feature to the HildonPannableArea allowing center on the child
widget receiving the focus.

* hildon/hildon-pannable-area.[ch](hildon_pannable_area_set_focus_child)
(hildon_pannable_area_center_on_child_focus ,hildon_pannable_area_class_init)
(hildon_pannable_area_get_property, hildon_pannable_area_set_property)
(hildon_pannable_area_button_press_cb, hildon_pannable_area_button_release_cb)
(hildon_pannable_area_[gs]et_center_on_child_focus)
Redefined GtkContainer set-focus-child signal, in order to center
properly on a child when it receives the focus. Defined a new property
"center-on-child-focus" in order to activate this feature (deactivated
by default). Public get/set methods defined to manage the property.

Fixes: NB#108487 (Phone settings dialog should scroll to focused field)

Signed-off-by: Alejandro G. Castro <alex@igalia.com>