Make wasWindowGeometrySaved() function consider new key format

The key format is changing in
frameworks/kconfig!184; take
that into account here.

Currently this function copies code from KWindowConfig, which is not
ideal; a more robust solution would be to expose the config file key
value publicly using a function in KWindowConfig that Konsole can call.
That's KF6 material, though, so for now, let's fix this issue by simply
supporting the new key format as well as the old one.