standard installation (FHS, XDG etc.)

• drop generation and usage of radiant version files (RADIANT_MAJOR, etc.)
• optional installation following Filesystem Hierarchy Standard
  • FHS install can be done by passing -DFHS_INSTALL=ON to cmake
  • radiant look dynamically for ../lib/netradiant/{plugins,contrib}
  • radiant look dynamically for ../share/netradiant/
  • can be installed systemwide by passing -DCMAKE_INSTALL_PREFIX=/usr to cmake
  • allow distributions to package NetRadiant
  • legacy Win95-like messy build still supported and used by default
• rename radiant binary to netradiant, obsoletes !92 (closed)
• XDG desktop integration, obsoletes !56 (closed)
  • desktop file
  • application icon
  • map mime type: thanks to !55 (closed) people can double-click .map files!
• strip .[ExecutableType] entirely (including preceding dot) outside of Windows
  • keep backward compatibility with existing build menus