PagerSelectExtender::getElement() allows consuming code to know the pager id used by a query

A new method PagerDefault::getElement() now allows to know the pager id used by a query, so that consuming code can avoid overriding it for other queries.

Before:

$query = db_select(..)->extend('PagerDefault');
print $query->element; // will throw an error "Cannot access protected property"

After:

$query = db_select(..)->extend('PagerDefault');
print $query->getElement(); // will print the pager ID

Therefore it can be now used as:

$build['pager'] = array('#theme' => 'pager', '#element' => $query->getElement());