Can not change channel from staff-text

Commit 37cb28d.
Could not change channel in "Staff Text Properties" window.

Reproduce:
1. Create a new score with piano staff.
2. Put "Instrument change" text to the second measure, pick "Violin" in it's properties.
3. Put "Staff Text" to the 3rd measure, go to it's properties, "Channel Change" tab.

Expected behaviour: There is "normal", "tremolo" and "pizzicato" in the Channel combo box.
Actual behaviour: There is only "normal".