Fix various issues with the dive add / edit manual dive code

- get_gas_from_events does NOT always set o2/he. It only updates them IFF
  a matching event is found; so we need to make sure we start out with a
  valid gas mix
- the way we tried to restore the edited dive in case of an edit to a
  manually added that is cancelled was completely bogus. Way too complex
  when we can simply and reliably simply store the dive and then copy it
  back

Fixes #270

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

qt-ui/diveplanner.h

@@ -61,7 +61,8 @@ public slots:
 	void createTemporaryPlan();
 	void deleteTemporaryPlan();
 	void loadFromDive(dive* d);
-	void undoEdition();
+	void restoreBackupDive();
+
 signals:
 	void planCreated();
 	void planCanceled();
@@ -73,6 +74,7 @@ private:
 	Mode mode;
 	QVector<divedatapoint> divepoints;
 	struct dive *tempDive;
+	struct dive backupDive;
 	void deleteTemporaryPlan(struct divedatapoint *dp);
 	QVector<sample> backupSamples; // For editing added dives.
 	struct dive *stagingDive;