New value placeholder for NS and ID

[moz0817]

Feature Description

I like to use header files to automatically render struct data for multiple pages.
For me, an additional placeholder (e.g. $NSID$) would be very useful. On start pages it behaves like $NS$, and on wiki pages it behaves like $ID$.

This means I can write filter: foo *~ $NSID$ in the header and get all the namespace hits on start pages, and only the corresponding ones from wiki pages.

The required changes in are minimal. Unfortunately I have no idea about push requests otherwise I would have done that.
File: meta/SerachConfig.php
Function: applyFilterVars()

global $conf; // ++++++++++
...
array(
'$ID$',
'$NS$',
'$NSID$', // ++++++++++
...
),
array(
$INFO['id'],
getNS($INFO['id']),
preg_replace('/' . $conf['start'] .'$/', '', $INFO['id']), // ++++++++++

The query on start pages can probably also be implemented differently.