Add VisualEditor timing data reporter

This is a copy-paste job, based on navtiming.py. This sort of barefaced code
duplication is something I try to avoid, so I tried extending EventLogging's
plugin architecture to cover the use-case of simple metric reporting modules in
Id8596535c. But I'm not convinced I got the design right, so I am reluctant to
merge that change. So instead, this.

Change-Id: I5838ff77458eff4080e5bcff526ead410b6795d5