Set incubation controller when a Window{} is loaded via QQmlApplicationEngine

This was the one convenience that was lost when transitioning templates
from QQuickView + Item{} to QQmlApplicationEngine + Window{}.

As the default window incubation controller was tied to the first
window's frameSwapped, we could easily run into a situation where
a secondary window required incubation while the first window
was idle. This would then starve the incubation controller. Instead
make it so that the renderloop emits "timeToIncubate" once it
is done with a renderpass over all windows, so the incubator
gets to run once and exactly once per vsync when animating.

The incubator logic was also flawed in that it could post
a lot of events to itself as a result of incubatingObjectCountChanged
and thus starve system events while processing incubation requests.
Now we start a timer and don't start it again until we have
completed an incubation pass.

Task-number: QTBUG-31203
Change-Id: Iea9e2c81efb46bb7875c70ccda0cdc4b3b3e58e7
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Add future compatibility note

File selectors are being delayed from releasing at the same time as
QQmlApplicationEngine, but will need to be enabled by default
(in QQmlApplicationEngine only) when they do arrive. Adding a note to
the documentation to help forwards compatibility.

Change-Id: Ia47a1da1afebd0da3bd1a97e7e4c8ee85f70b49a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>