combobox: Reuse code rather than reimplementing it

combobox: Reuse code rather than reimplementing it

We already have cell_layout_is_sensitive() to get whether at least one
cell in a Layout is sensitive, which we need because CellLayout/View
do not implement foreach(). So, since we wrote that, we can use it to
check our CellArea too, instead of doing foreach with a custom callback.