x11: Prefer kwinApp()->x11Connection() over connection()

Vlad Zahorodnii requested to merge work/zzag/no-connection into master

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

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