Merge pull request #18201 from etiennesegonzac/instacallscreen

Bug 990003 - Introducing a dedicated callscreen app for better performance r=rik, alive, 21
(cherry picked from commit 7afce8a6e965a45f38988227828ccdfb315394f3)

Conflicts:
	apps/communications/dialer/js/call_log.js
	apps/system/js/attention_screen.js
	apps/system/js/bootstrap.js
	apps/system/style/attention_screen.css
	apps/system/test/unit/bootstrap_test.js
	build/csslint/xfail.list
	build/jshint/xfail.list
	build/webapp-zip.js