Add Qt 5 support

If requested, build against Qt 5 instead of Qt 4.
For both builds the CMake minimum requirement is raised to 2.8.11,
that 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.