store and restore trees linked to annotation rows in Jalview projects

Step 0. Create a test in Jalview2XMLTests that writes a secondary structure tree to a project, and checks it is still linked to the annotations when imported.
Step 1. Work out how to export and import newick files that can be linked to annotation rows.
Step 2. Update jalview.xsd to store/restore:
- reference to annotation row in each tree leaf
- provider button/line display state
- anything else ?
Step 3. Fix code so the test pass :)
Bonus: make newick file export from the GUI useful for people too ? Perhaps also consider including pdb ids ?