fix read notification in QWindowsPipeReader::waitForPipeClosed

In QWindowsPipeReader::waitForPipeClosed we must check for available
bytes in the internal buffer and trigger the notified signal.

This fixes tst_QLocalSocket::writeToClientAndDisconnect on Windows.

Change-Id: I0f4d6cd73a0a8eac5b438b82984457068a9551d1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
QMacNativeWidget: Fix background rendering

The widget's background is transparent, but Qt is not owning the
backing store in this case, so we must make sure it gets properly
cleared and flushed.

Task-number: QTBUG-19840
Change-Id: I1087ce80aae3620d64a8c180129d79b5b022750b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
QPA: Adding API to support QWidgetAction on Mac

Includes the Cocoa implementation.

Task-number: QTBUG-19840
Change-Id: Id33bc8053b82116cf76ed591b6df823df3aef9bc
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>