Fix colum visibility selection in the divelist

Several changes:
- split the reload of the DiveListView from the reload of the header
- don't include the column title in the name of the setting; the title
  will change depending on the units and localization chosen by the user
- rename the slot that toggles visibility to make the code more readable
- use setCollumHidden() method to simplify the code
- don't save the width of hidden columns (as they would be saved as zero
  width and can then no longer be enabled)

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

qt-ui/divelistview.h

@@ -31,7 +31,8 @@ public:
 	void reload();
 
 public slots:
-	void hideColumnByIndex();
+	void toggleColumnVisibilityByIndex();
+	void reloadHeaderActions();
 
 Q_SIGNALS:
 	void currentDiveChanged(int divenr);