Replace wxTreeItemAttr and wxListItemAttr with wxItemAttr

The two existing structs were completely identical, just replace them with a
single wxItemAttr.

Notice that wxDataViewItemAttr is not quite the same, although pretty similar,
so it remains separate for now. It would be nice to combine it with this one
too in the future, e.g. to make it simpler to make items bold in a wxListCtrl.

include/wx/itemattr.h

@@ -0,0 +1,65 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/itemattr.h
+// Purpose:     wxItemAttr class declaration
+// Author:      Vadim Zeitlin
+// Created:     2016-04-16 (extracted from wx/listctrl.h)
+// Copyright:   (c) 2016 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_ITEMATTR_H_
+#define _WX_ITEMATTR_H_
+
+// ----------------------------------------------------------------------------
+// wxItemAttr: a structure containing the visual attributes of an item
+// ----------------------------------------------------------------------------
+
+class wxItemAttr
+{
+public:
+    // ctors
+    wxItemAttr() { }
+    wxItemAttr(const wxColour& colText,
+               const wxColour& colBack,
+               const wxFont& font)
+        : m_colText(colText), m_colBack(colBack), m_font(font)
+    {
+    }
+
+    // default copy ctor, assignment operator and dtor are ok
+
+
+    // setters
+    void SetTextColour(const wxColour& colText) { m_colText = colText; }
+    void SetBackgroundColour(const wxColour& colBack) { m_colBack = colBack; }
+    void SetFont(const wxFont& font) { m_font = font; }
+
+    // accessors
+    bool HasTextColour() const { return m_colText.IsOk(); }
+    bool HasBackgroundColour() const { return m_colBack.IsOk(); }
+    bool HasFont() const { return m_font.IsOk(); }
+
+    const wxColour& GetTextColour() const { return m_colText; }
+    const wxColour& GetBackgroundColour() const { return m_colBack; }
+    const wxFont& GetFont() const { return m_font; }
+
+
+    // this is almost like assignment operator except it doesn't overwrite the
+    // fields unset in the source attribute
+    void AssignFrom(const wxItemAttr& source)
+    {
+        if ( source.HasTextColour() )
+            SetTextColour(source.GetTextColour());
+        if ( source.HasBackgroundColour() )
+            SetBackgroundColour(source.GetBackgroundColour());
+        if ( source.HasFont() )
+            SetFont(source.GetFont());
+    }
+
+private:
+    wxColour m_colText,
+             m_colBack;
+    wxFont   m_font;
+};
+
+#endif // _WX_ITEMATTR_H_