Consistently check for type mismatch in all ports in wxDataViewCtrl.

Move the checks for the type mismatch between the type of the value returned
by wxDataViewModel and the type expected by wxDataViewRenderer into common
code. This avoids duplicating the same code in wxGTK and wxOSX and, more
importantly, means that this check is also performed in wxMSW when using the
generic version, so that the problems such as the one fixed in 3ff8c3c ("add
missing wxDataViewDateRenderer::GetDefaultType()") would be visible there too.

include/wx/dvrenderers.h

@@ -121,7 +121,27 @@ public:
 
     wxString GetVariantType() const             { return m_variantType; }
 
-    // helper that calls SetValue and SetAttr:
+    // Prepare for rendering the value of the corresponding item in the given
+    // column taken from the provided non-null model.
+    //
+    // Notice that the column must be the same as GetOwner()->GetModelColumn(),
+    // it is only passed to this method because the existing code already has
+    // it and should probably be removed in the future.
+    //
+    // Returns false if the value is missing (or invalid, i.e. has a wrong type
+    // differing from GetVariantType() of this renderer, in which case a debug
+    // error is also logged as it indicates an error in the user code).
+    bool PrepareValue(const wxDataViewModel* model,
+                      const wxDataViewItem& item,
+                      unsigned column);
+
+    // Prepare for rendering the given item by both calling PrepareValue() and
+    // setting up the attributes.
+    //
+    // This is currently only used in the generic version but should really be
+    // used everywhere, i.e. SetEnabled() and SetAttr() should be overridden in
+    // the native versions instead of adding platform-specific equivalents for
+    // them as it's done currently.
     void PrepareForItem(const wxDataViewModel *model,
                         const wxDataViewItem& item, unsigned column);
 
@@ -178,6 +198,12 @@ protected:
     // Called from {Cancel,Finish}Editing() to cleanup m_editorCtrl
     void DestroyEditControl();
 
+    // Helper of PrepareValue() also used in StartEditing(): returns the value
+    // checking that its type matches our GetVariantType().
+    wxVariant CheckedGetValue(const wxDataViewModel* model,
+                              const wxDataViewItem& item,
+                              unsigned column) const;
+
     wxString                m_variantType;
     wxDataViewColumn       *m_owner;
     wxWeakRef<wxWindow>     m_editorCtrl;