#82 Make corpus "save as XML" interruptible

[Johann Petrak]

When saving a corpus to XML (i.e. saving all the documents in the corpus as XML) in the GUI, a dialog window is shown that has a disabled "Stop" button. For very large corpora, saving as XML can take a very long time and should be interruptible. Since this saves in a document by document fashion, interrupting between documents should not cause any problems.

[Mark Greenwood]

Moved to github https://github.com/GateNLP/gate-core/issues/47