doc Callback in `Json.addListener` can't be a function

Since it's very untypical, it should be emphasised.

Also, documented when `responseJSON` is available.

Close #1010.
Close #1012.
feat #999 - Added new asserts methods

assertNull, assertNotNull, assertUndefined, assertNotUndefined, assertTruthy, assertFalsy

Close #1004.
fix #1020 Adding properties to the atflatskin for the MAC

Adding the properties in order to display the MultiAutoComplete widget with the atflatskin.
fix - old opacity value is stored in fullValue property
feat #1017 MultiAutoComplete navigation

It is now possible to use the left and right arrow keys to navigate throughout the whole widget, including currently inserted options. It is also possible to use the tab key to jump form the inserted options list to the text input field, and then from this field to the next focusable element in the page (outside the widget).

Close #1017