Update qsynth to 0.6.2
- A scalable (.svg) icon version has been added.
- Make man page compression reproducible.
- Ditching deprecated QTime methods for QElapsedTimer's (in compliance to Qt >= 5.14.0).
Test Plan: Connect to midi keybaord and play some tunes.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Differential Revision: https://dev.getsol.us/D8576