Update to Unity 6.3 to take benefit of performance improvements

[Tyroonis]

Description

The latest versions of Unity include several improvements to performance and the build process, as well as some updates for VR.
There’s also a security fix.

We should check whether there are any major changes and whether everything still works after the update.

TODOs

• Initial test
  • Known Errors
  • Build time
  • APK size
  • Build performance
• Update to Unity 6.3 LTS
• Check it out
  • Critical Bugs ?
  • Are there any difficulties with updating?
  • Build time
  • APK size
  • Build performance