Improve debugging interface

[cornerman]

We have OutwatchTracing, but we should improve on the debugging capabilities. We want to be able to hook a function into the outwatch patching process, such that you can measure the time it takes to build the virtual dom and how much time a snabbdom patch process takes.

We might need some global variable like OutwatchTracing.debugEnabled = true to not have any overhead in production from the debugging code.

[fdietze]

And maybe macro-generated source-code positions (Filename:Line) that get written as attributes into the dom (only enabled in fastOptJS). This allows to see the sourcecode position where the element comes from.

[cornerman]

Would be cool to inject an error reporter at the OutWatch.render* method - instead of the global OutwatchTracing.

[fdietze]

If in fastOptJS, add an attribute to every tag in the vdom:

Like this: <div sc="Table.scala:152" ... >