adding focus direction for gengrid

adding focus direction for gengrid

Summary:
It is implementation of focus_direction method for gengrid widget.

We need it in our project. We must service focus by
directions and we want to use gengrid widget. Additionally we must
support changing focus by keyboard (TV remote).
Unfortunately gengrid hasn't default implementation to present its
sub-objects as candidates for focus direction.

Our solution can search for focusable sub-objects (from realized items)
and change then focused and last selected variables in gengrid. If this
candidate wins then object will receive focus.

Reviewers: cedric, raster

CC: raster, seoz
Differential Revision: https://phab.enlightenment.org/D449