kcms/effects: Take into account system-wide defaults

Nicolas Fellarequested to merge
work/nico/effect-defaults into master

It's possible to provide system-wide default values in /etc/xdg/kwinrc

We need to take these into account when writing values and handling the defaults button.

When writing a value that matches the system-wide default, use revertToDefault().

When pressing the defaults button, reset to the system-wide default

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