Add Qt 5 support

Autodetect the Qt version to use, preferring Qt 5 over Qt 4.
Users can force a Qt 4 build passing -DUSE_QT4:bool=ON to CMake.

For both builds the CMake minimum requirement is raised to 2.8.11,
which provides automoc support. This means that automoc4 support is
removed with this patch.

Qt 5 builds require ECM and use KDEInstallDirs to install libraries,
while Qt 4 builds keep using InstallSettings.

The CMake module now defines POLKITQT-1_INCLUDE_DIRS with the include
directory just like Qt 5 modules does.  It contains the same value
as POLKITQT-1_INCLUDE_DIR.