preserve loaded file name for Save/Save As/Save A Copy file name proposals

See commit comment. It covers cases where input file folder is not valid anymore but the new file was read.
This is a second try (first after paths reworking).
 It is important for my usage case ( I've even nagged DiapDealer to get a meaningful file name from KindleImport plugin for this), but I think it is a general enhancement for Sigil. .