Revamping the build system (again...)

* LINGUAS can now be a comma-separated list, and it now supports quassel_xx_YY.ts style properly
* Get rid of the options for having builtin icons
* Instead, -DEMBED_DATA=ON will embed all external files into the binary, whereas all files
  will be installed externally if the option is not set
* Data files and icons are now installed into sane locations (needs to be checked on Mac)
* Various improvements and simplifications (yes, I've discovered PARENT_SCOPE now :D)

Note: - Code changes to find files again follow in the next commit
      - -DEMBED_DATA=OFF not supported on Win32 yet, and untested on Mac and for static builds
      - -DWITH_KDE=ON implies -DEMBED_DATA=OFF