Expose ACTION_FOO, ICON_TYPE_FOO and KEY_CODE_FOO to addons

This exposes
```
xbmcgui.KEY_BUTTON_FOO
xbmcgui.ACTION_FOO
xbmcgui.ICON_TYPE_FOO
```
to the addon API. This is on Spheres wishlist: *8.1 actions by name in onAction()/constants for action IDs* 

I wasn't sure if I should split Key.h into two parts to avoid the ```#ifndef SWIG``` but this seemed less intrusive