[Gengrid] Refactoring gengrid item show reigion calculation

[Gengrid] Refactoring gengrid item show reigion calculation

Summary:
Gengrid _item_show_region function calculate gengrid x, y moving postion with scroll to type.
Belows are summaries of this refactoring works.

1. Refactoring recalculation of scroll type.

2. Bug fix :
   - when x or y is 0, y or x value is set as default value, it->position-1.
   - when x or y is less then 1, calculation is not correct.
   - make calculate correct position of y when even though scroll direction is horiziontal
     (when virtical, x is also calculation correct position).

3.  change smart data variable s_type to scroll_to_type to make unity on gen widget.
     (genlist smart data variable name is scroll_to_type);

Reviewers: seoz, Hermet, raster

Reviewed By: raster

CC: bluezery
Differential Revision: https://phab.enlightenment.org/D474