BUGFIX: handle missing cache dir

fixed a nullpointer exception that only appeared when the cache
directory name was invalid.
locale fix for svg transforms

makes sure that the output locale for svg transform attributes is always
english. Otherwise, the application has problems properly displaying the
plots when running on computers with a different language.
Reduced complexity for sequence highlighting.

The sequences in the alignment are still highlighted, but by an
algorithm that is expected to have a better performance.