Make "Save As...." Interruptible

[greenwoodma]

When saving a corpus (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 can take a very long time and should be interruptible.
This should be easy to add for DocumentExporter instances as each document is handled independently via the GUI. Might be harder if the exporter is actually a CorpusExporter as the whole corpus is handed off to the exporter in one go.