Changes in mixer do not affect instruments if "Use Jack MIDI" enabled

MuseScore 2, commit ba34515.

Steps to reproduce:
1. Enable Preferences->I/O-> "Use Jack MIDI".
2. Create a new piano score, add a few notes.
3. Change sound to "Celestia" in Mixer.
4. Hit "Play".

Expected behavior: notes play with celestia sound.
Actual behavior: notes play with piano sound.

Due to the implementation, the same should happen to volume, pan, reverb and chorus settings.

This happens because we don't update channels' init list immediately after changing parameters in mixer. When we hit "Play" we init instruments and rewrite its' values with old parameters.

This bug occurs only if "Use Jack MIDI" option enabled.