text-list widget improvements

[graste]

The HtmlTextListAttributeRenderer does support the widget options max_count in the view_templates.xml files, but doesn't fall back or default to the TextListAttribute options with the same name. This should be done.

In addition to this the javascript widget should be changed to prevent adding the backspace plugin as that doesn't make sense when exactly one item can be selected from the list. Also the multiple=true on the Selectize is not necessary if max_count is 1.