CMake: switch back to kdsingleapplicationguard

- Reports from downstream Manjaro that the DBus activation doesn't
  work at all. Switch the default back to the old way, until it
  can be debugged more.

CMakeLists.txt

@@ -54,7 +54,7 @@ option( BUILD_TESTING "Build the testing tree." ON )
 option( WITH_PYTHON "Enable Python modules API (requires Boost.Python)." ON )
 option( WITH_PYTHONQT "Enable next generation Python modules API (experimental, requires PythonQt)." OFF )
 option( WITH_KF5Crash "Enable crash reporting with KCrash." ON )
-option( WITH_KF5DBus "Use DBus service for unique-application." ON )
+option( WITH_KF5DBus "Use DBus service for unique-application." OFF )
 
 # Possible debugging flags are:
 #  - DEBUG_TIMEZONES draws latitude and longitude lines on the timezone