Introduce a lock that blocks relayouts and config writes

The resize of the layout area can happen either by screen resolution
change or available screen area change (a panel appears or is resized)
This is not an atomic operation, as width and height are usually set in
2 different operations, and even worse the layout area is resized to
  match the available one with an animation, so many intermediate resizes
that should never cause a relayout happen.
In normal operation an event compression timer limits the actual
relayouts to hopefully one, but if the system is really slowed down
(for instance, startup) the timer may expire and cause relayouts in
non useful sizes, losing the needed configuration
In combination with

The lock blocks all relayout and config writes when the size of the
layout area doesn't correspond to corona availablescreenrect, which are
the only "settled" cases.

BUG:413645