Proposal for Save As.../Open dialog

[Geert Janssens]

See
http://www.kobaltwit.be/gnucash/images/SaveAsXml.jpg
http://www.kobaltwit.be/gnucash/images/SaveAsPostgres.jpg
http://www.kobaltwit.be/gnucash/images/OpenFile.jpg
http://www.kobaltwit.be/gnucash/images/OpenPostgres.jpg


These are 4 screenshots of a reworked File Access dialog.

What changed:
* The dialog gets a proper title
* If the selected backend is a file backend, the Database Frame is hidden
* And the other way around as well.
* Set a minimum width. Without it the dialog comes up too narrow.
* Some minor layout changes (spacing, justification,...) to harmonize with
  other GnuCash dialogs.

There's a glitch in the SaveAs screenshots (the Save As... button has the 
wrong label), but I have already fixed it.

Before I check in, I would appreciate some opinions on these changes.
I myself am not really sure "Storage Format" is a proper term to use.

Geert