partial implementation of sequence track copy

partial implementation of sequence track copy
this includes the code to track the portion of a sequence track that
has been selected, and provides a MussaWindow option to "copy" the selection
as a fasta file.

However the selection doesn't go to the clipboard yet, and the
MussaAlignedWindow also needs the option.

The actual Qt Slot that calls the glseqbrowser::copySelectedTrackAsFasta
is in the SequenceBrowserWidget.