v2.60_pre FMOD Sound / Restructuring

This might end up as v3.0 when some beta testing on LiveTraffic is done, but is stable enough if you want to try starting using the new features:

• Sound support by FMOD Core API by Firelight Technologies Pty Ltd. There are a few strings attached as there are licensing and attribution requirements to be followed. Please read the documentation on Sound.
• The XPMP2 repository has been restructured:
  • Can be used as GitHub submodule now and included in CMake builds using add_subdirectory
  • Please read the updated Build documentation
  • Other separate targets have been pushed out into their own repos, namely XPMP2-Sample (as a Public Template) and XPMP2-Remote, which make use of XPMP2 as a submodule