Buildsystem fixes

This series of commits provides

- cleanups of the cmake stuff
- small improvements
- installation of the application icon (referenced by the .desktop file) using macros copied by kdelibs

While the usage of the kdelibs macro is certainly not mandatory, it simplifies and automates a bit the actual cmake stuff required to install the icons.
Also, the minimum CMake required is now 2.8.5 (which has been released ~2 years ago, 2011-07-08), I hope it is not too high.