Use system DPI for some native Windows dialogs

Some native dialogs do not scale correctly (color picker, font picker,
open file with custom controls). ALl other native dialogs do scale correctly
(open file, open directory, find replace, print).

Change the DPI Awareness Context temporarily to SystemAware, so Windows handles
the scaling.

include/wx/msw/private/dpiaware.h

@@ -0,0 +1,72 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        wx/msw/private/dpiaware.h
+// Purpose:     AutoSystemDpiAware class
+// Author:      Maarten Bent
+// Created:     10/6/2016
+// Copyright:   (c) Maarten Bent
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_MSW_DPI_AWARE_H_
+#define _WX_MSW_DPI_AWARE_H_
+
+#ifndef WX_PRECOMP
+    #include "wx/msw/missing.h"
+#endif
+
+#include "wx/dynlib.h"
+
+#if wxUSE_DYNLIB_CLASS
+
+// ----------------------------------------------------------------------------
+// Temporarily change the DPI Awareness context to System
+// ----------------------------------------------------------------------------
+
+class AutoSystemDpiAware
+{
+    #define WXDPI_AWARENESS_CONTEXT_UNAWARE      ((WXDPI_AWARENESS_CONTEXT)-1)
+    #define WXDPI_AWARENESS_CONTEXT_SYSTEM_AWARE ((WXDPI_AWARENESS_CONTEXT)-2)
+    typedef WXDPI_AWARENESS_CONTEXT
+            (WINAPI *SetThreadDpiAwarenessContext_t)(WXDPI_AWARENESS_CONTEXT);
+
+public:
+    AutoSystemDpiAware()
+        : m_prevContext(WXDPI_AWARENESS_CONTEXT_UNAWARE),
+          m_pfnSetThreadDpiAwarenessContext((SetThreadDpiAwarenessContext_t)-1)
+    {
+        if ( m_pfnSetThreadDpiAwarenessContext == (SetThreadDpiAwarenessContext_t)-1)
+        {
+            wxLoadedDLL dllUser32("user32.dll");
+            wxDL_INIT_FUNC(m_pfn, SetThreadDpiAwarenessContext, dllUser32);
+        }
+
+        if ( m_pfnSetThreadDpiAwarenessContext )
+        {
+            m_prevContext = m_pfnSetThreadDpiAwarenessContext(
+                                WXDPI_AWARENESS_CONTEXT_SYSTEM_AWARE);
+        }
+
+    }
+
+    ~AutoSystemDpiAware()
+    {
+        if ( m_pfnSetThreadDpiAwarenessContext )
+        {
+            m_pfnSetThreadDpiAwarenessContext(m_prevContext);
+        }
+    }
+
+private:
+    WXDPI_AWARENESS_CONTEXT m_prevContext;
+
+    SetThreadDpiAwarenessContext_t m_pfnSetThreadDpiAwarenessContext;
+};
+
+#else // !wxUSE_DYNLIB_CLASS
+
+// Just a stub to avoid littering the code with wxUSE_DYNLIB_CLASS checks.
+class AutoSystemDpiAware { };
+
+#endif // wxUSE_DYNLIB_CLASS/!wxUSE_DYNLIB_CLASS
+
+#endif // _WX_MSW_DPI_AWARE_H_