API Provide a thin alternative to loadPanel/submitForm.

This is needed in some situations when we only want to update a
small single component, sometimes even using a different controller to
the one implied in the URL.

An example here is reloading dynamically the subsite dropdown without
reloading the entire page, updating a filter sidebar or suchlike.
Merge pull request #2352 from mateusz/loadfragment

API Provide a thin alternative to loadPanel/submitForm.