Mute music preview during calibration

[jkemperman]

Uses message events to inform the main window when calibration has started, and when the calibration window closes. Actual pausing/resuming is handled by the MusicSelectPreviewer component.

Fixes #449

[codecov-io]

Codecov Report

Merging #507 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #507   +/-   ##
======================================
  Coverage    84.1%   84.1%           
======================================
  Files         171     171           
  Lines        5387    5387           
  Branches        1       1           
======================================
  Hits         4531    4531           
  Misses        856     856

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3d1ecd2...2d66e4f. Read the comment docs.

[dtinth]

Thanks a lot for your contribution!

Merging this now :)