Support "subtracting" ROIs

[dscho]

In ImageJ 1.x, you can hold down the Alt key when selecting a region which will _subtract_ the selected part from the current selection.

This requires a substantial extension of our current ROI framework.

[dscho]

Unfortunately, JHotDraw does not offer any functionality like that, so most likely I will have to write code to generate a JHotDraw figure from an AWT GeneralPath and use AWT for the geometry operations.

[dscho]

Progress: I found out how to convert a Java AWT-type Shape object (given its PathIterator) into a JHotDraw-type BezierPath.

Since JHotDraw depends on AWT's geometry classes anyway, we can now rely on java.awt.geom.Area to do the constructive area geometry (such as subtraction). We will also be able to use this to flatten multiple selections into one single composite one.

As always, progress can be seen in the Git repository (in this case, the 'roi-subtract' branch).

[dscho]

The real problem seems to be that we need to add support for multi-path bezier RegionOfInterests to ImgLib2...

[dscho]

And related to the lack of multi-path bezier ROIs in ImgLib2: there is no support for such a beast in JHotDrawCanvas either. So I'll do that, too.

[curtis]

(In #1022) Selection tool is successfully eliminated. Alt key will not work until subtracting ROIs works (see ticket #878). But there is nothing left to do for this specific ticket.

[curtis]

Finishing this work will have to wait till beta2, as we are too short on time.