preserve current file name for future save as actions if appropriate.

Preserves current file name as proposal for, well, file name, by  Save As action if currentFilePath is no longer valid.
This is something of a "must have" feature for me - so if it not acceptable just point me in the right direction.  I've tried to be minimalistic, I think the home folder should be set as starting point - or, better yet, the last valid folder?