Remove code duplication

Define DATA_PATH in the root CMakeList.txt since modules might need it (for example the http intf or the lua module)

reported by Felipe Manzano and Anibal Sacoo from Core Security Technologies.

contrib: Setting DYLD_LIBRARY_PATH is bad because it results most of the time in library mismatch. This is an old hack from the past which shouldn't be needed anymore.

cmake: Use CPack to create the package. (Use `make package` or `cpack`). Should produce a tgz/tar.gz/rpm/dmg/nsis. This probably need some tuning.

cmake: Enable testing. (Launch with `make test` or with `ctest` (see ctest --help)). Plus minor cleanup.

FindFFmpeg.cmake: Perform the ffmpeg search only once. (That's probably not the cmake's spirit... to check).

Qt4 - SoundSlider. Enable MouseTracking and show a tooltip with the volume that it would be if you would click there. Similar to the main slider behaviour.

      Remove the QDockWidget, this widget is clearly not what we want, since it is a Qt::Tool and stay above the video, it doesn't want to grow in the MI and it doesn't record correctly its size. Moreover, this force the GNOME/Cleanlooks hack that is boring on Xfce, KDE4 and e17.
      Fix the resize when the video is done.
      Fix the qt-start-minimize bug
      Rename a few functions and separate the code functions.
      Simplify a few resize/update call.