optionally preserve '-' and '.' gap characters when importing alignments

'-' and '.' are used in the results of HMM searches to represent two different types of gap.
'-' refers to a deletion state in the hmm model.
'.' refers to an insertion state in the hmm model.

When importing the result of an HMM search, Jalview's normal behaviour of replacing . and - with the current gap character should be suppressed, and a user preference provided to prevent replacement when importing results from the local file system.