Creating an entry point for notebook manager extensions

Following the comments about  #5989, I'm turning my multicolumn notebook manager hack into an extension. I have the feeling that the only way to load extensions as early as the notebook manager is to create a new entry point by:

- Firing app_initialized.DashboardApp event when loaded the notebook manager (in tree/js/main.js)
- Updating tree.html template to load nbextensions through custom.js and this newly created event.

Is this change necessary and acceptable to you?
The 4 space a re markdown formatting. please avoid removing, or code will not be highlighted. cf : 
http://ipjsdoc.herokuapp.com/classes/IPython.customjs.html