Bug 1018086 - [tarako] Need a way to dispatch 'headset-button' system message to engineering mode app r=alive

Target: to dispatch hardware-button and headset-button message via IAC from system to engmode app

Design:
1\register the 'headset-button' message in system manifest.webapp (note: the IAC keyword --- 'validationtools-request' had been merged into the v1.3t before, see https://bugzilla.mozilla.org/show_bug.cgi?id=1003636);

2\put all handlings of IAC in the file validationtools_helper.js, to monitor these messages and dispatch to engmode if needed