Don't add relative paths in the RECENT_FILES history

If Jalview is launched with parameters
  open <filepath>
then <filepath> is added to the RECENT_FILES history list.
If this is a relative path (e.g. "examples/uniref50.fa") then it is disabled for picking in the file browser unless the correct parent folder is selected.
This can be mildly annoying.
Solution is to ensure that only full paths are added to the history list.