fix #312 Hard-coded dropdown skin class for the List in the MultiSelect and similar widgets
fix #315 AutoComplete JS error when clicking the dropdown button

Fixing a regression after b1eb1323439bc3f3203ef5b0c3880410105e8715 using
the similar solution as in 1c8d106816f790fa65465660ee9a6b9ab375af50.

Pull request #316.
fix #304 expected JSON response in RequestMgr

All request handler wipe out the failure response from the server. This prevents REST applications.
Moreover, using JSON the expected response should contain JSON also in case of errors.