Cleanup: Turn widget accessor-functions into simple pointers

The keeps track of different sub widgets needed by other parts
of the code, notably:
	MainTab
	PlannerDetails
	PlannerSettingsWidget
	ProfileWidget2
	DivePlannerWidget
	DiveListView
Access to these widgets was provided with accessor functions.
Now these functions were very weird: instead of simply returning
pointers that were stored in the class, they accessed a data
structure which describes the different application states.
But this data structure was "duck-typed", so there was an
implicit agreement at which position the pointers to the
widgets were put inside. The widgets were then down-cast by
the accessor functions. This might make sense if the individual
widgets could for some reason be replaced by other widgets
[dynamic plugins?], but even then it would be strange, as one
would expect to get a pointer to some base class.

Therefore, directly store the properly typed pointers to the
widgets and simply remove the accessor functions. Why bother?

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/mapwidget.cpp

@@ -103,9 +103,9 @@ void MapWidget::selectedDivesChanged(QList<int> list)
 {
 	CHECK_IS_READY_RETURN_VOID();
 	skipReload = true;
-	MainWindow::instance()->dive_list()->unselectDives();
+	MainWindow::instance()->dive_list->unselectDives();
 	if (!list.empty())
-		MainWindow::instance()->dive_list()->selectDives(list);
+		MainWindow::instance()->dive_list->selectDives(list);
 	skipReload = false;
 }