Add a new User::getDisplayName() to return the name that should be displayed in the...

Add a new User::getDisplayName() to return the name that should be displayed in the interface.
Add a UserDisplayName hook to allow extensions to give custom display names for users.
Add a $wgRealNameInInterface to use the real name of a user as the display name.
To start of the first use of the display name functionality tweak SkinTemplate to declare the userdisplayname and use it inside of personal_urls.