Export CMake targets

Improve CMake configuration files by exporting targets and put version
information in a dedicated file.

Library is exported with its full path thus making it usable by other
projects. Relative paths are used to make it relocatable.

For more information see:
  http://www.cmake.org/Wiki/CMake/Tutorials/Packaging
  http://www.cmake.org/Wiki/CMake/Tutorials/Exporting_and_Importing_Targets
  http://www.cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file

While we are at it, rename library to QtXdg for Qt 4 builds and Qt5Xdg
for Qt 5 builds.