KWindowConfig: create hasSavedWindowSize() and hasSavedWindowPosition()

These allow apps to know whether there's saved window size or position
data, for whatever reason they might need to know that.

Konsole is an example of an app that wants to know this, and exposing
these functions allows it to get rid of some code that needs to be kept
in sync, which has been troublesome in the past because it's an app and
not on the Frameworks release schedule.