Unit export, edit song title & comment

New features

• Open to export song into separate files for each unit
• Dialog for editing song title and comment

Fixes

• Change the shortcut for toggling whether a parameter is copied or not from C to Shift+C
  • C was too easy to accidentally press as part of 'Ctrl+C' for copy
• Tweak UI a bit so that various widget heights are aligned, and more widgets are disabled before initial project load (thanks @EwanGreen4 or the contribution)
• Fix a crash if a stylesheet uses a custom font
• Fix incorrect help text regarding transposition / parameter tweaking (thanks @PureAsbestos for the report!)