Combobox: remove properties

[wkeese]

I think we can get rid of the hasDropDownArrow and autoFilter properties, in order to remove the combinations of settings we don't want to support or test:

• hasDropDownArrow=true, minFilterChars=1
• hasDropDownArrow=false, minFilterChars=0
• hasDropDown=false, autoFilter=false

Instead define behavior that:

• minFilterChars=-1: <input> is readonly, and clicking it opens the dropdown with all the choices.
• minFilterChars=0: Original behavior, where user can open the dropdown at any point, but can also do filtering by typing something in the <input>.
• minFilterChars >= 1: AKA "autocomplete" mode. Don't show the down arrow icon because we don't want to let the user see all the possible completions. Instead, force user to type n characters before filtering occurs.

... or something like that.