more refactors to the Notifier classes

Move the autoloc list toggling out into its own notifier class.

Move the function that echos the error on the current line out into its
own notifier.

A few other changes were required in the process:
* move s:WideMsg() out into syntastic#util autoload lib so it is
  available from the cursor notifier.
* move s:ShowLocList() into the LocList class so it is available for
  the autoloclist notifier
* move s:HideLocList() into Loclist as a class method mainly to keep the
  two show/hide methods together
* move the s:old_line var into the Cursor notifier where it is needed