Sync activities after kwin restart

Activities are loaded async. During this time any fetch of activity information is incorrect as we will treat any settings as invalid.

We need to ignore attempts to set activities during this time, but also refresh Window's concept of activities once we are loaded.

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[x11window] Remove duplicate activities check

We check if activities are valid in Window::setActivities. We don't need to do it when we read the X11 properties.