Add framework for Per-Monitor DPI Awareness on Windows

React to the WM_DPICHANGED event and update the size of the  child windows and
the top-level window. Scale the minimum and maximum window size to the new DPI.

Only react to WM_DPICHANGED when DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 is
used.

include/wx/defs.h

@@ -2830,6 +2830,7 @@ WX_MSW_DECLARE_HANDLE(HBITMAP);
 WX_MSW_DECLARE_HANDLE(HIMAGELIST);
 WX_MSW_DECLARE_HANDLE(HGLOBAL);
 WX_MSW_DECLARE_HANDLE(HDC);
+WX_MSW_DECLARE_HANDLE(DPI_AWARENESS_CONTEXT);
 typedef WXHINSTANCE WXHMODULE;
 
 #undef WX_MSW_DECLARE_HANDLE

include/wx/msw/missing.h

@@ -23,7 +23,11 @@
 #endif
 
 #ifndef WM_PRINTCLIENT
-    #define WM_PRINTCLIENT 0x318
+    #define WM_PRINTCLIENT      0x0318
+#endif
+
+#ifndef WM_DPICHANGED
+    #define WM_DPICHANGED       0x02E0
 #endif
 
 #ifndef DT_HIDEPREFIX

include/wx/msw/toplevel.h

@@ -165,6 +165,9 @@ protected:
                                           int& x, int& y,
                                           int& w, int& h) const wxOVERRIDE;
 
+    // WM_DPICHANGED handler.
+    bool HandleDPIChange(const wxSize& newDPI, const wxRect& newRect);
+
     // This field contains the show command to use when showing the window the
     // next time and also indicates whether the window should be considered
     // being iconized or maximized (which may be different from whether it's
@@ -191,6 +194,14 @@ protected:
     wxWindowRef m_winLastFocused;
 
 private:
+    // Keep track of the DPI used in this window. So when per-monitor dpi
+    // awareness is enabled, both old and new DPI are known for
+    // wxDPIChangedEvent and wxWindow::MSWUpdateOnDPIChange.
+    wxSize m_activeDPI;
+
+    // This window supports handling per-monitor DPI awareness when the
+    // application manifest contains <dpiAwareness>PerMonitorV2</dpiAwareness>.
+    bool m_perMonitorDPIaware;
 
     // The system menu: initially NULL but can be set (once) by
     // MSWGetSystemMenu(). Owned by this window.

include/wx/msw/window.h

@@ -586,6 +586,10 @@ public:
     // Should be overridden by all classes storing the "last focused" window.
     virtual void WXDoUpdatePendingFocus(wxWindow* WXUNUSED(win)) {}
 
+    // Called from WM_DPICHANGED handler for all windows to let them update
+    // any sizes and fonts used internally when the DPI changes.
+    void MSWUpdateOnDPIChange(const wxSize& oldDPI, const wxSize& newDPI);
+
 protected:
     // this allows you to implement standard control borders without
     // repeating the code in different classes that are not derived from