EditTableDialog: Reload the current DB schema when cancelling

All the editing in the EditTableDialog, that is the DBBrowserDB class to
be specific, messes up the current representation of the DB schema. This
is fine when the changes are actually committed by clicking the OK
button but causes confusion when the dialog is cancelled. None of the
changes are actually written to the database but the UI looks like they
are.

This closes issue #74.