Filter DOM load/save deferreds through ve.track callback

Binding ve.init.mw.Target#onLoad and the anonymous function which calls ve#track
with timing data to the resolution of the same deferred makes the timing
measurement sensitive to how long it takes ve.init.mw.Target#onLoad to yield,
which in turns depends on whether the requisite ResourceLoader modules for
setting up the editing surface are ready. This muddles the definition of what
we're actually measuring, making it dependent on a race condition.

This patch moves the tracking call to a filter function through which the jqXHR
deferred is threaded. This ensures that the 'duration' datapoint on the domSave
and domLoad events captures just the time spent talking with the API.

Change-Id: I6e162014a4043d4ff9422131ae87fb25d0ab4c29
Merge "Filter DOM load/save deferreds through ve.track callback"