inspector: Fix prop-list search

inspector: Fix prop-list search

Instead of handle_event(), use set_key_capture_widget().

Also, use the fact that we now propagate key events throughout the
stage, and connect to the GtkInspectorPropList instead of the toplevel.
We only want the search bar to be active while that widget is mapped
and able to receive events.

And also, handle the "search bar" visibility on ::search-started.