coremodel: Fix empty search view stopping music playback

active_core_object(self, value) currently tries to set the player model from the search result any time the search is active. This causes the empty search view to stop playback of a song previously started from the search view as described in #440.

Added a check so that the player model is only set from the search result if the search is active and has state RESULT, allowing playback to continue under the empty search view as expected.

Fixes #440