Don't use QAbstractButton::pixmap() as this will always return 0.

Instead of using pixmap() we use icon() instead which is what the
extension menu is after in this case.

Task-number: QTBUG-38301
Change-Id: I2c238b50b0fcf04318100d2d6d39f563ef3eff5b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
QMdiArea: Ignore events of other instances in the event filter.

This caused for example the variable QMdiAreaPrivate::isSubWindowsTiled
being cleared from another instance, which resulted in odd
resizing behavior.

Task-number: QTBUG-31448
Change-Id: I880f7e415d6654e52499df1bfaf99f2c55d86320
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
(cherry picked from qtbase/9e2c4bc67f30eb1e8d776afe3a092d5994aaab38)