FoxVox Update v3.6

Changelog:

• Fix variable operations for null/empty values
• Fix problem with variable incrementation during evaluation
• Fix intermittent unhandled exception caused by simultaneous variable change
• De-initialize Library on close for variable disengagement and removal
• Prevent variables set via outputs from registering updates
• Enable entering axis curve point coordinates explicitly on right-click context menu
• Add PTT blocking support

This mainly updates a very stable release with a few extra features, giving some performance benefits as well.  It also fixes an issue with making course/heading changes not working correctly in the BMS library.