Overlay annotations, ROI export and ESWC import

This addresses:

12
9, namely ESWC import and the ability to export paths as ROIs
On exit, XZ/ZY images are only discarded if they have not been modified during the session
Several GUI tweaks and improvements that make the plugin more usable on small screens:
- Visibility of tracing panes can be toggled at will, and brought together via a "Arrange panes" command
- Most commands will now work when a pane has been closed
- Footprint of Path list window has been reduced

fiji / SNT