Expose configFileString() publicly

Normally you want to use one of the higher level APIs that does all this stuff for you, but if you have a custom implementation of window state saving and restoration, it's useful to be able to save that data under the same config schema as everything else.

Konsole does this, for example, and exposing this function for it allows it to get rid of some code that needs to be kept in sync with this, which has been troublesome in the past because it's not on the Frameworks release schedule. See utilities/konsole!893 (merged).