Adds in keyboard navigation to dropdown menus (and navbar dropdowns), suitable for both keyboard-only users and screen reader-users.

Keyboard handling based on boostrap's V4 code for dropdown at:
https://github.com/twbs/bootstrap/blob/v4-dev/js/src/dropdown.js
and closely follows the W3 ARIA best practices standard:
https://www.w3.org/TR/wai-aria-practices-1.1/#kbd_general_within

This works similar to native <select> elements and OS menus:

• When the menu is opened, the first .dropdown-item or .dropdown-header is focused (.dropdown-dividers are ignored)
• Pressing cursor up/down will navigate the enabled menu items. Disabled items will be skipped.
• Pressing ESC while the menu is open/focused will close the menu and return focus to the menu toggle button.
• Hitting TAB while the menu is open/focused will close the menu and go to the next (or previous, if the SHIFT-TAB is pressed) focusable item on the page.

dropdown-items now have the ARIA role="menuitem"

The .sr-only toggle text (for split dropdowns) can now be set via the prop toggleText on <b-dropdown> for better language/locale support. Defaults to "Toggle Dropdown".

Notes:

• The original bootstrap V4 code ignores dropdown-headers for keyboard navigation, although I have added them in as otherwise they are not "visible" to screen reader users. Headers might provide more context to the items below them:

<div class="dropdown-menu" role="menu">
  <h6 class="dropdown-header">User</h6>
  <a class="dropdown-item" href="#">Delete</a>
  <a class="dropdown-item" href="#">Show</a>
  <h6 class="dropdown-header">Group</h6>
  <a class="dropdown-item" href="#">Delete</a>
  <a class="dropdown-item" href="#">Show</a>
</div>

• The original Bootstrap V4 dropdown code didn't automatically focus the first item in the dropdown. This PR diverges from Bootstrap's code and more closely follows the W3 best practices.
• Bug fix: Menu toggle buttons now respect the disabled attribute. If you had a disabled menu, clicking the button would still open it. This is now fixed

Please review at your leisure.