[D7] Provide method to get the pager's element ID

Description of the need

This is a cross-port from https://www.drupal.org/node/702778, and part of the task list #5770. The D7 issue added a method, getElement(), to PagerDefault that returns the element ID of the pager. Having this method allows developers an easier way to have more than one pager in the same page without conflicts.

Before this new method, the only way to handle more than one pager in the same page was to hard-code the element ID through PagerDefault::element(). This doesn't make much sense and can lead to unexpected conflicts if two different modules need to add a pager to the same page. PagerDefault already has a way to avoid those conflicts by keeping track of IDs in a static variable, but until this feature was added in D7, it was impossible to get the internally assigned element ID (which is a protected property).

PR coming.