QTimer: don't circumvent <chrono> safety net

By templating on the <chrono> types and unconditionally using
duration_cast to coerce the duration into a milliseconds, we
violate a principal design rule of <chrono>, namely that non-
narrowing conversions are implicit, but narrowing conversions
need duration_cast. By accepting any duration, we allow non-
sensical code such as

   QTimer::singleShot(10us, ...)

to compile, which is misleading, since it's actually a zero-
timeout timer.

Overloading a non-template with a template also has adverse
effects: it breaks qOverload().

Fix by replacing the function templates with functions that
just take std::chrono::milliseconds. This way, benign code
such as

    QTimer::singleShot(10s, ...)
    QTimer::singleShot(10min, ...)
    QTimer::singleShot(1h, ...)

work as expected, but attempts to use sub-millisecond
resolution fails to compile / needs an explicit user-
provided duration_cast.

To allow future extension to more precise timers, forcibly
inline the functions, so they don't partake in the ABI of the
class and we can later support sub-millisecond resolution by
simply taking micro- or nano- instead of milliseconds.

Change-Id: I12c9a98bdabefcd8ec18a9eb09f87ad908d889de
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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