Partial fix for deferred properties when combined with components

Current deferred properties implementation did not store context
or compiled data pointers correctly. Those pointers are now stored
when the defer is reached, so as to avoid confusion (confusion leads
to asserts or crashes).

Does not extend the deferred property support to allow multiple
deferred blocks per item. This now prints and error and the side
effect is only that one of the deferred blocks is lost. This use
case is sufficiently rare that it may not be worth the cost.

Task-number: QTBUG-30325
Change-Id: I80cb074ed4452e95020208a0142a91e721bced7d
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
qmlscene: don't call show() on the window if it's already showing

That way Component.onCompleted() can optionally do showMaximized()
without being overridden.

Task-number: QTBUG-31260
Change-Id: I6f5c3025efd3b38809be85da4172241695168a6c
Reviewed-by: Nikita Krupenko <krnekit@gmail.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>