Add showClear property to Dropdown

[brian428]

The new clear icon for Dropdown is a nice feature, but I don't think it should just automatically be applied. We have plenty of dropdowns where a value must be chosen, so the new clear icon is confusing and also breaks our logic (before the clear icon, there was no way for the user to select "nothing").

I would say make the clear functionality opt-in via an input property. Or, less desirable would be an input property to turn it off (but that would mean we'd have to manually go through and disable it throughout our code).

[ASK83]

I came across the similar issue (#5286) and making it optional would solve that one too.

[FirstVertex]

Agree with @brian428 that this should be opt-in.

In addition, please consider adding a css class to the parent ui-dropdown whenever the clear icon is present (suggest ui-dropdown-clearable). That way my app's css can adjust, or even hide, the presentation of the clear icon based on the parent class, instead of having to target all dropdowns.