Add GtkStyleContext helper class

Manages ownership of GtkStyleContext and GtkWidgetPath, provides functions
for creating and querying style contexts.

include/wx/gtk/private/stylecontext.h

@@ -0,0 +1,46 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/gtk/private/stylecontext.h
+// Purpose:     GtkStyleContext helper class
+// Author:      Paul Cornett
+// Created:     2018-06-04
+// Copyright:   (c) 2018 Paul Cornett
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GTK_PRIVATE_STYLECONTEXT_H_
+#define _WX_GTK_PRIVATE_STYLECONTEXT_H_
+
+#ifdef __WXGTK3__
+
+class wxGtkStyleContext
+{
+public:
+    wxGtkStyleContext();
+    ~wxGtkStyleContext();
+    wxGtkStyleContext& Add(GType type, const char* objectName, ...) G_GNUC_NULL_TERMINATED;
+    wxGtkStyleContext& Add(const char* objectName);
+    wxGtkStyleContext& AddButton();
+    wxGtkStyleContext& AddCheckButton();
+    wxGtkStyleContext& AddHeaderbar();
+    wxGtkStyleContext& AddLabel();
+    wxGtkStyleContext& AddMenu();
+    wxGtkStyleContext& AddMenuItem();
+    wxGtkStyleContext& AddTextview(const char* child1 = NULL, const char* child2 = NULL);
+    wxGtkStyleContext& AddTooltip();
+    wxGtkStyleContext& AddTreeview();
+    wxGtkStyleContext& AddTreeviewHeaderButton(int pos);
+    wxGtkStyleContext& AddWindow(const char* className2 = NULL);
+    void Bg(wxColour& color, int state = GTK_STATE_FLAG_NORMAL) const;
+    void Fg(wxColour& color, int state = GTK_STATE_FLAG_NORMAL) const;
+    void Border(wxColour& color) const;
+    operator GtkStyleContext*() { return m_context; }
+
+private:
+    GtkStyleContext* m_context;
+    GtkWidgetPath* const m_path;
+
+    wxDECLARE_NO_COPY_CLASS(wxGtkStyleContext);
+};
+
+#endif // __WXGTK3__
+#endif // _WX_GTK_PRIVATE_STYLECONTEXT_H_