Add a Timeout error API to the serialport class

Hmm, we will have slight issues with the Timeout error I am now working on because unfortunately we do not seem to have defined the "UnknownError" as -1 even though QFileDevice also has "UnspecifiedError" in the middle of the list.

Perhaps, we can change the documentation order compared to the header file content to have the UnknownError last in the list. It will be more convenient for the end users, but less for us, developers. I think it is still worth it, just like when the code snippets are mixed. This is unfortunate, but that is how it is anyway.

See the following links for details:

http://doc-snapshot.qt-project.org/qt5-
stable/qtcore/qprocess.html#ProcessError-enum

http://qt-project.org/doc/qt-5.0/qtcore/qfiledevice.html#FileError-enum

http://doc-snapshot.qt-project.org/qt5-stable/qtnetwork/qabstractsocket.html#SocketError-enum