Ease usage of buttons with dropdown

[jelhan]

<BsDropdown> component provides an intuitive API for having links in a dropdown:

<BsDropdown as |dd|>
  <dd.toggle>Dropdown <span class="caret"></span></dd.toggle>
  <dd.menu as |ddm|>
    <ddm.item>
      <ddm.linkTo @route="index">Something</ddm.linkTo>
    </ddm.item>
  </dd.menu>
</BsDropdown>

But it does not provide a good API for having buttons in a dropdown. Using a button currently looks like this:

<BsDropdown as |dd|>
  <dd.toggle>Dropdown <span class="caret"></span></dd.toggle>
  <dd.menu as |ddm|>
    <ddm.item>
      <button class="dropdown-item" type="button" {{on "click" this.doSomething}}>Action</button>
    </ddm.item>
  </dd.menu>
</BsDropdown>

Yielding a <ddm.button> similar to the existing <ddm.linkTo> would make the API more ergonomic.