Sonic Visualiser updated to v2.1

Sonic Visualiser has been updated to version 2.1 for Windows, Mac OS X and Linux.

Changes:

• Fix incorrect handling of FixedSampleRate outputs (Vamp SDK fix).
• Make it easier to see results from transforms that return a single point.
• Make labelling clearer on time-value layers.
• Add discrete-curve mode for time-value layers (for curves with gaps).
• Update code to build against Qt5.