toolbarlayout: Buffer calls to calculateImplicitSize()
calculateImplicitSize() is called by ToolBarLayoutDelegate to update
the implicit size after the delegate knows what its actual size is.
However, since it needs the delegates to determine the size, it will
call createDelegates() to update the list of delegates. This works
fine if the delegate is properly async, unfortunately in some cases Qt
will force the delegate creation into sync mode as there is no async
incubation controller. This then creates an infinite recursion as
ToolBarLayout does not yet know that the delegate exists and tries to
create a new one.
To avoid that, rather than calling calculateImplicitSize() directly,
queue a call and keep track of whether we queued it. This means we only
re-run it after all sync creation is done and in addition it means we
only run it once instead of for every delegate. It may even help the
async case as that still has a chance of creating multiple delegates in
one go.