Loading a gzipped file and saving it leaves the new file uncompressed

Import a gzipped file (now recognized by content - JAL-2656) and use the 'File->Save' option to save the file. Jalview has no way of knowing that the file was imported from a gzip stream, so doesn't offer to compress again.

Fix is to have Jalview remember whether the file should be gzipped on save (and or provide an option to override this behaviour).