cmake: make use of GNUInstallDirs for the install dirs

Use the GNUInstallDirs CMake module to get the installation directories
for binaries and XDG data, instead of defining them manually.

This bumps the CMake requirement to 2.8.5, as GNUInstallDirs has been
introduced in that version.
cmake: move stuff to the top-level

Move to the top-level the search for Icecream, the setup of the Qt
definitions and the include directories.
cmake: improve/simplify FindIcecream.cmake

Make use of the find_package_handle_standard_args CMake macro, and its
advanced mode (introduced in CMake 2.8.3) to simplify the
FindIcecream.cmake code.
Also, take the icecc version out of the pkg-config result and export it
as LIBICECREAM_VERSION.