x11: Prefer kwinApp()->x11Connection() over connection()
With connection(), we will look up the x11 connection property on kwinApp() object, which is less efficient than just calling a method on the app object.
note that we cannot simply do s/connection()/kwinApp()->x11Connection()/ as there are parts of kwin that need to lookup connection as a property on the app object (mostly tests)
x11 backend was left unchanged