Prepare build system for KDE Frameworks support

Besides setting everything up for finding frameworks, this commit also
changes the handling of install paths. We now support the standard CMake
variables of the form CMAKE_INSTALL_<TYPE>_DIR, which can be overridden
on the command line. If KDE integration is enabled, the default paths set
by KDE (4 or frameworks) will be used.

Some additional cleanups were done in passing, too.