Fixed agenda views menu
Fixing headline serialize for multiple content lines
Revamped the agenda plugin system

* Moved headline caching back into agenda view manager, so
  goto_headline, change, undo etc also are generalized and views only
  have to expose what the content of the view should be and nothing
  else. They can optionally have a setup where they could setup extra
  mappings or other things needed by the plugin.
* Added notes view, it lists all notes.
Added refiles agenda view

Shows all headlines in the refile file to be refiled