Optionally always offer default file type when exporting

These changes are in response to issue #7401 and add an option on the "Edit/Preferences/Image Import & Export" page to cause the default export file type to be offered whenever exporting a file (rather than only using the default file type when a new file has been created or an xcf file opened).

This option is of use if GIMP is being used to edit a number of (say) png files when the output files all have to be (say) in tiff format.

When the option is not selected (which is the default setting) the current behaviour is preserved.