Allow wxDataViewCtrl to gain focus in wxOSX

Fix numerous focus-handling bugs in wxOSX in combination with
NSScrollView (which cannot get focus by itself and which was already
treated specially in many, but not all, places), including

- inability to set the focus
- loss of wxEVT_SET_FOCUS events
- loss of wxWindow<->NSView association after clearing wxDVC columns

src/osx/cocoa/window.mm

@@ -2328,12 +2328,18 @@ bool wxWidgetCocoaImpl::GetNeedsDisplay() const
 
 bool wxWidgetCocoaImpl::CanFocus() const
 {
-    return [m_osxView canBecomeKeyView] == YES;
+    NSView* targetView = m_osxView;
+    if ( [m_osxView isKindOfClass:[NSScrollView class] ] )
+        targetView = [(NSScrollView*) m_osxView documentView];
+    return [targetView canBecomeKeyView] == YES;
 }
 
 bool wxWidgetCocoaImpl::HasFocus() const
 {
-    return ( FindFocus() == m_osxView );
+    NSView* targetView = m_osxView;
+    if ( [m_osxView isKindOfClass:[NSScrollView class] ] )
+        targetView = [(NSScrollView*) m_osxView documentView];
+    return ( FindFocus() == targetView );
 }
 
 bool wxWidgetCocoaImpl::SetFocus()
@@ -2341,6 +2347,10 @@ bool wxWidgetCocoaImpl::SetFocus()
     if ( !CanFocus() )
         return false;
 
+    NSView* targetView = m_osxView;
+    if ( [m_osxView isKindOfClass:[NSScrollView class] ] )
+        targetView = [(NSScrollView*) m_osxView documentView];
+
     // TODO remove if no issues arise: should not raise the window, only assign focus
     //[[m_osxView window] makeKeyAndOrderFront:nil] ;
     [[m_osxView window] makeFirstResponder: m_osxView] ;