Implement mw.requestIdleCallback for deferred background tasks

Implement mw.requestIdleCallback for deferred background tasks

We often use the idiom "window.onload" or "$(window).on('load')".
Since code loads asynchronous, this is problematic because the event won't
always be observed as it may fire before the event handler is attached.

Most tasks also don't really want to wait until the page is loaded (in which
case it would run immediately if the page is already loaded). Rather their intent
is just to defer it to a later point in time – to avoid disrupting user events.

Bug: T111456
Change-Id: Ieba0440c6d83086762c777dfbbc167f1c314a751