GDK W32: Implement show_window_menu()

This is achieved by sending undocumented message WM_SYSMENU
to the window.
Before doing that, the window is given WS_SYSMENU style
(to enable window menu) and some combination of
WS_MAXIMIZEBOX (for "Mazimize" item)
WS_MINIMIZEBOX (for "Minimize" item)
WS_SIZEBOX (for "Size" item)
depending on which operations are currently permissible.

WM_SYSMENU is processed by DefWindowProc(), which results
in showing the window menu. We remove extra styles
at the first opportunity (WM_INITMENU message), as they
alter the way our window is rendered.

https://bugzilla.gnome.org/show_bug.cgi?id=763851

gdk/win32/gdkprivate-win32.h

@@ -97,6 +97,18 @@
 #define WM_MOUSEHWHEEL 0x20E
 #endif
 
+/* According to
+ * http://blog.airesoft.co.uk/2009/11/wm_messages/
+ * this is the actual internal name MS uses for this undocumented message.
+ * According to
+ * https://bugs.winehq.org/show_bug.cgi?id=15055
+ * wParam is 0
+ * lParam is a pair of virtual desktop coordinates for the popup
+ */
+#ifndef WM_SYSMENU
+#define WM_SYSMENU 0x313
+#endif
+
 #ifndef CF_DIBV5
 #define CF_DIBV5 17
 #endif