(#26) Intelligent Import Autofill

Started some experimentation on making the import table editable by
assigning each pane it's own document object (with a few minor tweaks).

This also gives import action plugins a document object through which
they can make their changes before it is progressed onto the working
document.
(#26) Intelligent Import Autofill

The import document was moved from being a property of each pane
to a property of the entire import window.  This seemed to be the
most consistent with the way it was structured prior.  All tests
pass in this commit.