Refactor roughCount into an atomic QGadget, fix usage of i18np

i18np's first argument should be a valid number. Meanwhile roughCount
was a string. Instead we should pass the original number as the
plural-deciding argument, while using the formatted string only for the
actual substitution.

It is only natural to factor out the counter into a gadget class of its
own, since all of its properties (and their semantics like the
`roughCount.length > 0` special case) change atomically and thus do not
require separate connections.

Also, let's store the Timer inline, instead of allocating dynamically.

BUG: 476148

(cherry picked from commit 576e460a)
(and adapted to Qt 5)