keyboard: Improve Empty Search State.

The empty search state looks out-of place due to it being nested inside a listbox. This is because we can only detect empty states through gtk_list_filter_func().

Refine it by making the empty search placeholder a child of the main stack. Introduce an additional helper function to detect when all the listbox rows would be hidden by the filter.

As a part of #1222 (closed)

The method isn't very elegant, but it works :)
cc @ids1024 @bertob @exalm. Please review.