Missions - update 3

The 3rd update upon the 0.4 Missions release, which ended up being more significant than a regular minor update:

• shields
  • all spacecrafts can have a shield equipped, which recharges after not suffering hits for a short time
  • related graphics, sound, game logic and HUD changes are all included
• control updates
  • speed holding can be toggled
  • tweaks to control details for better feel and more responsive control
  • added a small horizontal tracking and increased projectile velocity for the main player weapon, the plasma gun for easier aiming
• HUD updates
  • new HUD settings menu, with options to turn the new features on/off
  • target hull/shield bar appears above target
  • offset impact indicators instead of separate aim assist and impact indicators (off by default)
• mission and balance updates
  • player ship has more varied equipment across missions, and so detailed equipment info is added to the missions screen
  • difficulty levels now have more differences between them
  • rebalanced all existing missions, changed equipment and added messages
  • removed 5 static "missions" from the mission list
  • added 3 new (dynamic) missions
  • under the hood changes fixing, extending and simplifying features in the mission engine
• fixes
  • fixed white flickering on some phone browsers
  • fixed explosion graphics color issue
• editor updates
  • duplicate and move element buttons for array properties
  • unset button for optional properties
  • jump to reference button for reference properties
  • selection history support with back and forward buttons
  • shader preview (showing the final, assembled shader code with syntax highlighting)
  • showing information for spacecrafts and weapons
  • button and property tooltips
  • small tweaks and fixes
• other minor tweaks and fixes