Add support for multimedia keys to wxMSW and wxGTK

Add WXK_XXX constants for the standard multimedia keys and generate events
corresponding to them under wxGTK and wxMSW.

Closes https://github.com/wxWidgets/wxWidgets/pull/157

include/wx/defs.h

@@ -2695,7 +2695,25 @@ enum wxKeyCode
     WXK_SPECIAL17,
     WXK_SPECIAL18,
     WXK_SPECIAL19,
-    WXK_SPECIAL20
+    WXK_SPECIAL20,
+
+    WXK_BROWSER_BACK,
+    WXK_BROWSER_FORWARD,
+    WXK_BROWSER_REFRESH,
+    WXK_BROWSER_STOP,
+    WXK_BROWSER_SEARCH,
+    WXK_BROWSER_FAVORITES,
+    WXK_BROWSER_HOME,
+    WXK_VOLUME_MUTE,
+    WXK_VOLUME_DOWN,
+    WXK_VOLUME_UP,
+    WXK_MEDIA_NEXT_TRACK,
+    WXK_MEDIA_PREV_TRACK,
+    WXK_MEDIA_STOP,
+    WXK_MEDIA_PLAY_PAUSE,
+    WXK_LAUNCH_MAIL,
+    WXK_LAUNCH_APP1,
+    WXK_LAUNCH_APP2
 };
 
 /* This enum contains bit mask constants used in wxKeyEvent */