fix OutputDevice::edid()

This patch transports the EDID data base64-encoded over the wire.

Apparently, we can't just send random QByteArrays as "strings" over, it
has to be encoded and decoded. So...

* base64-encode the data before sending to the client
* base64-decode it on the client side
* document the above, fix documentation woes in the xml definition
* change test accordingly

The test data used was actually invalid, it's a base64 string of the
actual data, so fix the tests (which actually breaks it), and encode on
the server-side and decode on the client side.

REVIEW:126380