Extend the SpecialStatsAddExtra hook for a more distinct display of extra items

The current approach leaves not much room for a meaningful
display of varying extra statistical items nor its ability to display
a special page link as item name.

The change proposed allows for varying header titles as well as
item descriptions using the standard language parser. The proposed
change uses is similar approach as in the InfoAction hook. A item
now is specified as [<header-message>][<item-message>] = number

Use === array() check instead of a count() and continue legacy support
for the single array structure used prio 1.21.

Change-Id: I78107c72953878c86d221fb6f83baff5f6c11f09