Improve usage of the break state for the Tx line

At present use of the methods setBreakEnabled() and sendBreak()
is a little ambiguous and not clear, has not a common use case.

Besides these methods have various behavior on different platforms.
For example, in Windows, the method sendBreak() is implemented as
blocking through the Sleep() call; but at the same time, in *nix
this method is non-blocking. Besides, behavior of this method
in *nix variously for the Linux and BSD platforms (add url: ... ).

I offer to implement identical behavior of these methods and to
expand the use case:

• sendBreak():

To make asynchronous with use a QTimer in the
"single shot" mode. And to emulate through twice calls of the
method setBreakEnabled() with the true and false value. Also to
forbid a multiple call of the sendBreak() by user while
operation is in progress.

• setBreakEnabled()

To make as property and to add the symmetric method
isSetBreakEnabled(). Besides, to add the breakEnabledChanged
signal for the notification that the Tx line is switched to
the "break/nonbreak" mode. By analogy with DTR/RTS signals.

The main problem here is implementation of the isSetBreakEnabled()
method. Because it seems that in *nix doesn't exist an API for
determination of a status of Tx line (or the "break" mode on the
Tx line). Though, probably it can be done through the
SecondaryTransmittedDataSignal?

On Windows there is an possible to define a "break" mode since
there is I/O control code which can return an appropriate flag of
the "hold reason".

But anyway, even if it is impossible, then we can try get a
"break" mode indirectly after a successful call of
the method setBreakEnabled().