Implement background color attribute for wxDataViewCtrl in wxOSX

Add support for this attribute for text-like cells to the native macOS
version too, to bring it up to parity with the generic and GTK ones.

Closes https://github.com/wxWidgets/wxWidgets/pull/1673

include/wx/osx/cocoa/dataview.h

@@ -142,6 +142,7 @@ public:
 
         [m_origFont release];
         [m_origTextColour release];
+        [m_origBackgroundColour release];
     }
 
     NSCell* GetColumnCell() const { return m_ColumnCell; }
@@ -186,6 +187,7 @@ public:
     // ones that do.
     NSFont *GetOriginalFont() const { return m_origFont; }
     NSColor *GetOriginalTextColour() const { return m_origTextColour; }
+    NSColor *GetOriginalBackgroundColour() const { return m_origBackgroundColour; }
 
     void SaveOriginalFont(NSFont *font)
     {
@@ -197,6 +199,11 @@ public:
         m_origTextColour = [textColour retain];
     }
 
+    void SaveOriginalBackgroundColour(NSColor *backgroundColour)
+    {
+        m_origBackgroundColour = [backgroundColour retain];
+    }
+
     // The ellipsization mode which we need to set for each cell being rendered.
     void SetEllipsizeMode(wxEllipsizeMode mode) { m_ellipsizeMode = mode; }
     wxEllipsizeMode GetEllipsizeMode() const { return m_ellipsizeMode; }
@@ -226,6 +233,7 @@ private:
     // we own those if they're non-NULL
     NSFont *m_origFont;
     NSColor *m_origTextColour;
+    NSColor *m_origBackgroundColour;
 
     wxEllipsizeMode m_ellipsizeMode;