Tags support and editor.

[tinevez]

• Press the Y key to bring up the menu.

• As a temporary debug trick, press U to spit the tag set model in the console.

• [x] Undo mechanism should record changes to the TagSetStructure

• [ ] Integrate with Tobias JFrame menu framework.

• [ ] Remove the debug action (U key).

[tpietzsch]

Most of what was done in this PR is now in master.

JPopupMenu accelerators I also couldn't get to work. I switched back to the old "overlay" tagging menu.

Some of the TODO items still remain open, so I won't close this PR yet.

I reset the unify-views-tags branch to master. The old stuff is in unify-views-tags-backup, in case we want to refer back to JPopupMenu etc.

[tinevez]

We can kill this PR and the branch right?