Flag friends

Related to #27

Let's be slow to be sure we're on the same page... this is a pull request adding a new feature "Mark as favourite" that allows users to flag other users as favourites.
- Button "Mark/Unmark as favourite" on each user's profile page.
- Flagged users are displayed on each user's profile page under tab "Favourites" thanks to a view. Allow filtering on user's fullnames.
- Button "Favourites" in the welcome block.

Fixes/Improvements:
- Limit the access to user's own favourites only. At the moment the view doesn't show other users's favourites, but the tab is still displayed when visiting someone else's profile. Not possible to limit access of a view to the current user only (have to be hand-coded ?)
- Theme the flag links (on the favourites list of each user) as the others buttons. Recopy buttons's theme into the css theming this view ?

And a general "how-to-do-best" question, always the same:
This feature on itself doesn't work, it depends on modifications in wsuser.module and wsuser-member-actions-block.tpl.php among others, is it an expected way of developing ?