Fix #713: Add cleanupObservers flag to List

Add a flag, defaulted to true, to set whether to cleanup the observers. This is
used by the tree plugin, which will set it to false, so that the observers are
not cleaned up when the tree plugin is used. The child quries in the tree plugin
set the _numObservers value to > 1, which ends up making the List not refresh
the observer for the main query. Changes to the the main row data in the store
would not be reflected in the grid as a result.

Fixes #713