Do not print a warning for the serialport closure if it is not open

This is necessary because end users will get a warning needlessly if the serial
port is not open when using the library.

There are two alternatives:

* Close in the destructor only if it is open.
* Do not print a warning.

The latter approach is more consistent with the other QIODevice subclasses, so
it is better to stick to that for now.

Change-Id: I918c203b1f5c52fba20de1373c298b8527a0e5a1
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Revert the incompatible qwineventnotifier.h for Qt4

The change

* 50ab4d4a8ff5f882b9699cbde26ad9d8824a4824

sync'd up the header file in question with the updates, but those
changes are incompatible with Qt4. This unfortunately means, a mix
has to be applied from both version.

Tested on Windows with Qt4 (MinGW).

Change-Id: I0468c03ccfb276096cfc3adf80bfc0b3488c81f2
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>