Make hildon_touch_selector_column_get/set_column public

Make hildon_touch_selector_column_get/set_column public

* doc/hildon-sections.txt: Add the new public methods below.

* examples/hildon-picker-button-multicolumn-example.c:
(create_touch_selector):
* examples/hildon-touch-selector-example.c: (create_selector):
* examples/hildon-touch-selector-multi-cells-example.c:
(create_selector):
Use hildon_touch_selector_column_set_text_column()

* hildon/hildon-touch-selector-column.h:
* hildon/hildon-touch-selector.c:
(hildon_touch_selector_column_set_text_column),
(hildon_touch_selector_column_get_text_column):
Make hildon_touch_selector_column_set/get_column() public, use
them in the inline example, document them, and reference to them
in the docs.