OStreamMessageHandler : Prefix every line with message level

[johnhaddon]

This allows log parsers to correctly identify the type of each line for syntax highlighting and the like. The immediate motivation is for a new LocalJobs panel in Gaffer, that captures the output from LocalDispatcher subprocesses and turns it back into messages for display into a GafferUI.MessagesWidget.

[danieldresser-ie]

All looks reasonable. If I have any nitpick, it would be that looking at the sample output in the test case, it is a bit confusing to immediately associate the second line line of the message with the context of the first when there are a mixture of single line and multiline messages coming out together. I would consider adding an extra newline after any message that covers more than 1 line, but I don't feel strongly about this.

[johnhaddon]

I would consider adding an extra newline after any message that covers more than 1 line, but I don't feel strongly about this.

I also wonder about either repeating the context on each line, or indenting secondary lines by the length of the context, so they line up with the message on the first line. But I think this is good enough for now - see how you feel when you see it I the real world in the MessagesWidget in Gaffer, and we can consider any tweaks then.