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.
cmake: install the application icon

Copy part of the KDE4Macros.cmake module from kdelibs (just the parts
of it dealing with icons installation) as KDE4Macros-Icecream.cmake
(to avoid clash with the kdelibs-provided version), along with its
license.

Make use of kde4_install_icons to properly install the icons in the
global icon theme.
Merge pull request #5 from pinotree/buildsystem-fixes

Buildsystem fixes