fix: sointu-play should use cmd.MainSynther (#174)

This way you can choose if you want to use the native synth or the 
go synth by defining the build tag native.

CHANGELOG.md

@@ -50,6 +50,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Numeric updown widget calculated dp-to-px conversion incorrectly, resulting in
   wrong scaling ([#150][i150])
 - Empty patch should not crash the native synth ([#148][i148])
+- sointu-play does not default to the native synth yet, choose via `-tags=native`
 
 ### Changed
 - The keyboard shortcuts are now again closer to what they were old trackers