Bug 852162 - Generic reporting of events received during startup path

This introduces two standard events:
- above-the-fold-ready: when data on the first screen is ready
- startup-path-done: everything for the first panel is done

startup-path-done is mandatory to whitelist an app for that perf test
(cherry picked from commit c24074cc9be379334fecc5b83302d5adbc7b2806)

Conflicts:
	apps/communications/contacts/js/contacts_list.js
Bug 852162 - Fix a bad resolution of a merge.
(cherry picked from commit 16fecfd832c78c44774e6d8cfa48e2cb09b191c1)