fix #1976: optionally sort postfix queue result

fix #1976: optionally sort postfix queue result

The PostfixMailQueue widget uses an Ext.data.BufferedStore, due to
the potential size of the resultset, which does only support remoteSorting [0]

By adding two optional parameters ('sortfield' and 'sortdir') we can use
them for sorting the mailq output accordingly.

The sorting is kept in PMG::API2::Postfix instead of PMG::Postfix, because
sorting (as opposed to filtering) needs to happen after the complete result
is known, and there is no gain in pushing it further down.

[0] only mentioned in the source-code - not in the referencedoc

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>