Remaining features C++ REST Wrappers

The first patch-set implemented the most important enablers. Few features were omitted to keep commits smaller. This task is about adding these features:

• readyRead() handling *)
• settable timeout for requests
• progress signaling (download upload)

*) A note for readyRead() support

QRestReply class provides getter functions to often used datatypes:
byteArray() => QByteArray
text() => QString
jsonObject() => QJsonObject

It needs to be documented and defined what happens when these functions are called during a "readyRead()" handling, when potentially only part of the data is received.

Initial thinking:

• byteArray()
  • return all bytes that haven't been read yet (exhausts/consumes the data)
• text()
  • return QString received since start or previous text() call, internally keeping text codec state. The text data may be UTF-8, UTF-16 or UTF-32, which means that the character boundary may be byte-wise cut in the middle, causing a loss of character or an error => by keeping a text codec state this could be addressed (QStringDecoder).
  • Alternatively discourage and convert whatever has been received so far, and exhaust / consume the data
• jsonObject()
  • discouraged as the json object is probably not valid/complete. Attempt to convert to jsonObject, exhausts/consumes the data