feat(dropdown): use .value() for uibDropdownConfig

[onlywei]

This allows people to use .decorator() to customize this object.
#6004.

[wesleycho]

Note that the issue shouldn't be closed, as this should be done for every component.

[wesleycho]

Unfortunately, I'm going to have to revert this since it turns out this is a breaking change (the constant becomes no longer injectable inside a config function).

[onlywei]

@wesleycho why is that a breaking change? I grepped through the src/ directory and found that it was not being injected into any config functions.

[pkozlowski-opensource]

I grepped through the src/ directory and found that it was not being injected into any config functions.

But users of this lib might be doing so.

[onlywei]

I see. In that case, going forward, will the below be the recommended way for users of the lib to configure the dropdown library?

import angular from 'angular';
import uibDropdown from 'angular-ui-bootstrap/src/dropdown';

angular.module('MyApp', [uibDropdown])
  .config(function (uibDropdownConfig) {
    uibDropdownConfig.openClass = 'is-open';
  });

?