CMake buildsystem fixes

1) installation dirs
    a) can now install in /usr/local using -DCMAKE_INSTALL_PREFIX settings
    b) docs and mkspecs are installed into prefix/share
2) fix building the tests
3) new cmake option to force building against Qt4 even if Qt5 is found (same as GammaRay does)
4) new INSTALL-cmake.txt to document how to use the CMake buildsystem
5) some minor doc fixes