W32: correctly guess max window size from the size of the workarea of the screen it's on

Get monitor on which the most of the window is located (nearest monitor if
window is not on screen), get its work area (area not occupied by taskbar or
any other bars) and use that for maxsize.

Previous default of 30000 meant that windows maximized onto full screen,
even covering the area where taskbar is.

https://bugzilla.gnome.org/show_bug.cgi?id=726592

gdk/win32/gdkevents-win32.c

@@ -3076,8 +3076,27 @@ gdk_event_translate (MSG  *msg,
 	}
       else
 	{
-	  mmi->ptMaxTrackSize.x = 30000;
-	  mmi->ptMaxTrackSize.y = 30000;
+	  HMONITOR winmon;
+	  MONITORINFO moninfo;
+
+	  winmon = MonitorFromWindow (GDK_WINDOW_HWND (window), MONITOR_DEFAULTTONEAREST);
+
+	  moninfo.cbSize = sizeof (moninfo);
+
+	  if (GetMonitorInfoA (winmon, &moninfo))
+	    {
+	      mmi->ptMaxTrackSize.x = moninfo.rcWork.right - moninfo.rcWork.left;
+	      mmi->ptMaxTrackSize.y = moninfo.rcWork.bottom - moninfo.rcWork.top;
+	    }
+	  else
+	    {
+	      /* Apparently, this is just a very big number (bigger than any widget
+               * could realistically be) to make sure the window is as big as
+               * possible when maximized.
+               */
+	      mmi->ptMaxTrackSize.x = 30000;
+	      mmi->ptMaxTrackSize.y = 30000;
+	    }
 	}
 
       if (impl->hint_flags & (GDK_HINT_MIN_SIZE | GDK_HINT_MAX_SIZE))