Improve the build system; add new features and examples:

Improve the build system; add new features and examples:
* Add ability to build QtGstreamer as a static library by passing -DSTATIC_QTGSTREAMER=1 to cmake.
* Add a FindQtGstreamer.cmake script, which is installed as QtGstreamerConfig.cmake
  together with a generated QtGstreamerTargets.cmake script, that can be used to
  find QtGstreamer from external cmake-based projects. Both shared and static versions
  of QtGstreamer are supported.
* Add an uninstall target, copied from the KDE4 scripts.
* Rewrite the CMakeLists.txt of the examples so that they can also be built standalone
  and serve as examples of how to use QtGstreamer from external projects.
* Add qmake project files for the examples, to act as examples for people using qmake.