Fix dangling references

[pannal]

GHI (If applicable): #

Description:

Upon shutdown there are several references that can't be cleared, such as:

15:58:37.215 T:18584 WARNING: CPythonInvoker(3, C:\Users\panni\AppData\Roaming\Kodi\addons\script.plex\default.py): the python script "C:\Users\panni\AppData\Roaming\Kodi\addons\script.plex\default.py" has left several classes in memory that we couldn't clean up. 
The classes include: 
class XBMCAddon::xbmcgui::ListItem,
class PythonBindings::XBMCAddon_xbmcgui_WindowXMLDialog_Director,
class PythonBindings::XBMCAddon_xbmc_Player_Director,
class PythonBindings::XBMCAddon_xbmcgui_WindowXML_Director,
class PythonBindings::XBMCAddon_xbmcgui_WindowXML_Director,
class PythonBindings::XBMCAddon_xbmcgui_WindowXMLDialog_Director

This takes care of those references by setting ManagedListItem's dataSource to a dummy instance upon invalidation.

Checklist:

• [x] I have based this PR against the develop branch

[ruuk]

Thanks!