Update QML download API to match widgets and add documentation

Rename the download signal to downloadRequested and only start a
download if it has been explicitly accepted by the user, else cancel
it by default.
Replace the downloadPercentage property with totalBytes and receivedBytes
to also give the user information about download size.
Additionally this patch adds missing documentation.

Change-Id: I9d895386cf033f2efffe3ebac6f08f94c6fe0c19
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>
Reviewed-by: Jocelyn Turcotte <jturcotte@woboq.com>