Display slowness warning before opening a V2 file

This is somewhat invasive as aborting the XML file read requires us to
report things up the recursive parsing chain.

What we really need to do here is to ask the user how they want to use the
data from reverse geo lookup. But for now we only warn about the fact that
this can take a while.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.h

@@ -90,6 +90,7 @@ public:
 	void printPlan();
 	void checkSurvey(QSettings *s);
 	void setApplicationState(const QByteArray& state);
+	void showV2Dialog();
 	QUndoStack *undoStack;
 private
 slots: