Exact SLERP, File::executablePath(), standalone++

- launchers removed, standalone should be launched from bin/<platform>/openzone
- $ORIGIN-based rpaths for standalone => can be launched from any directory
- ozCore
  * String::EMPTY for const references where needed
  * implemented exact quaternion SLERP
  * File::executablePath()
- client
  * Client: executable path detection & default parameters adjusted so
    standalone can be run from any dir without additional parameters