Context item addons

This PR added a new Addon Extension point called "xbmc.context",
The xml for such an Extension point looks like:

```xml
<extension point="xbmc.context" library="addon.py" visible="{VISIBILITY_EXPRESSION}">
      <label>Label</label>
      <label lang="de">Beschriftung</label>
 </extension>
```

where you can do the same in VISIBILITY_EXPRESSION as you would do in in skins via:
```xml
<visible></visible> 
```

Also added a new expression: addon.hasSetting() where you can query for addon settings.

Possibilities could be:
- Make Scripts available when skins don't support them, eg. Home Theatre Experience, Actors Biography etc..
- http://forum.xbmc.org/showthread.php?tid=141982
Should probably think about where in all the `GetContextButtons` methods these calls are made. Each window is a bit different now. E.g. here, `item` can be null so the context menu can be opened on button and other gui elements.