select and deselect leaves bound to sequences hidden in current view affects group membership of displayed group

Steps to reproduce:

1. Create a tree on existing alignment.
2. hide some sequences.
3. partition tree and then select a group in the alignment that contains one or more hidden sequences.
4. colour the group by blosum62 or some other conservation-type colourscheme
4. click on an unselected leaf corresponding to one of the hidden sequences. The colouring of the group will change as if the hidden sequence were visible and being included/removed from group.

Fix: Should check if sequence is visible before adding/removing it from current selection group in response to tree leaf selection event.