Implement an extensible way to add buttons to the script editor

[imagejan]

To allow running scripts in batch from the script editor, we should have a Batch button next to the Run and Kill buttons in the script editor.
I did some initial work on the batch-processing-draft branch, but it would be much better if we had a way to provide this functionality in an extensible way, much the same as SearchActionFactory plugins in scijava-search.

How about creating a ScriptAction plugin type, so that the batch-processor project can provide a BatchScriptAction that results in a Batch button being added, and the script is run with the batch processor upon click?!

[imagejan]

The static implementation of a batch button is in #15 for now.