Default pager size as a setting instead of a constant

Default pager gives 50 items per page. It seems a bit limited on some installation with searches having several thousands of results most of the time.

It's possible to change it on the bottom of the pager but it could be handy to have this as a global setting as well. It could fit well in the Search preferences (civicrm/admin/setting/search?reset=1).

I think everything goes in CRM_Utils_Pager with ROWCOUNT constant but there might be other places that uses this 50 numbers without using the constant.