Emit a single change when changing track selection in the mixer.

Emit a single change when changing track selection in the mixer.

Previously changing track-selection emitted 2 "stripable_selection_changed"
events: select (nil); select (new).

This fixes an issue with the FaderPort ctrl surface when editing plugins:
Switching from one track to another follows selection, but if no track is
selected, the FP switches from "Edit Plugins" to "Track" mode.