Add SpecialPage::getGroupName and use it

Add SpecialPage::getGroupName and use it

Having all group mapping for Special:SpecialPages in the global
$wgSpecialPageGroups is not a good OO style.
Created a method SpecialPage::getGroupName, which than can be overridden
by each subclasses to the featured group name.

Added also SpecialPage::getFinalGroupName to get the groupname on
Special:SpecialPages to handle the customization and
to keep $wgSpecialPageGroups for b/c

Change-Id: I1de3a186f0a59ec5ecb8996c5f805cf164e9637f