Keep type ahead search box inside view widget

In order to be able to reuse them for tree-view and icon-view the following methods were moved:

exo_icon_view_get_work_area_dimensions was moved to exo_gtk_get_work_area_dimensions

exo_icon_view_search_position_func was moved to exo_gtk_position_search_box

Fixes #96 (closed)

@MShrimp4, if you have time to take a look