Add shortcut for the URL Shortener action

[lamblinc]

A nice improvement would be to have a keyboard shortcut for getting shortened url.

[oanalavinia]

The Alt+u shortcut has been added. Note that the shortcut keys can be modified by overriding the urlshortener.menu.action.shortcut translation. Check here the keys that are already used by XWiki

Documentation will be added on store once the new version is releases