orbtk_core: implement PagerState methods
orbtk_core: implement PagerState methods

Struct PagerState will provide an index to a pager instance.
Use methods to adopt this pointer index.

* get_current_index
* set_current_index

* lexicographical function sort

Signed-off-by: Ralf Zerres <a55c5beace021459791f5070a9a90148cd697b8f@networkx.de>

redox-os/orbtk