Toggle inclusion of '-' characters for un-annotated regions in annotation rows

When an AMSA file like:

  >#_efef
     X X

is imported (see JAL-686), it becomes an annotation row like:

  efef --X---X

This can look untidy, when the only symbols that were intended to be shown were 'X'.

The user should have the option of enabling/disabling the display of the '-' characters in annotation rows (which correspond to the current alignment gap character).