Bug fixes: Class deletion across sample & undo/redo for polygon+mag lasso

[davramov]

This PR fixes two issues:

1. Annotation classes were defined globally, so deleting from one sample would potentially delete from others that use the same classes. Also, it would pollute new samples with existing classes. This is now fixed, and labels are sample-dependent.
2. Undo/redo glitches with the polygon and magnetic lasso tools. Undoing to open a shape back up, and then redo would not close the shape correctly, and the UI buttons worked differently from the keyboard shortcuts. Also fixed.