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>

qt-models/filtermodels.cpp

@@ -644,7 +644,7 @@ void MultiFilterSortModel::myInvalidate()
 #if !defined(SUBSURFACE_MOBILE)
 	int i;
 	struct dive *d;
-	DiveListView *dlv = MainWindow::instance()->dive_list();
+	DiveListView *dlv = MainWindow::instance()->dive_list;
 
 	divesDisplayed = 0;