SequenceBrowser should handle the copying

SequenceBrowser should handle the copying
viva encapsulation, I pushed the code to handle copying sequence
down into the SequenceBrowser from SequenceBrowserWidget on the idea
that someone might want to use the inner widget on its own.

Hmm... maybe that's not reasonable? Not sure anymore. However
I do think that I want the popup menu to be tied to the innermost widget.