Allow adding (font-)icons to menu items

This was initially requested in #1841, but that issue was repurposed to add icons to the Admin Bar. This issue here is the reincarnation of that original request:

We should allow icons to be used in menus. For Drupal, the Icon API module includes the icon_menu submodule that allows:

1. Selecting an icon (from whatever font is provided by the Icon API). Perhaps we should implement it as an auto-complete search field instead of a huge drop-down though. 2. Selecting to position the icon before or after the menu item title. Perhaps we should also allow replacing the menu item title with the icon as 3rd option. Use case for that: the home icon on the admin bar. 3. Render the icon in breadcrumbs or not. 4. Choose to wrap the title in an element tag or not (div or span - 4a) and also define classes for it (4b).