Stabilize tst_qfiledialog.

selectFile(): Ensure dialog is destroyed before the temporary file
to prevent leak.

selectFiles(): Introduce scopes to prevent co-existence of 2
instances of QFileDialog, which causes warnings from file watchers
and occasional crashes in accessibility.

Discovered while investigating:

Task-number: QTBUG-38414
Change-Id: I94d2bb3e05538416286641cb08a88d8b3837a8a6
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Stabilize tst_qfilesystemwatcher::watchFileAndItsDirectory().

Use QTRY_COMPARE for dirChangedSpy, remove all unneeded
remove()/setPermission() calls (originating from the time
the test did not use QTemporaryFile), add more QVERIFY()
and messages for the other operations.

Task-number: QTBUG-30943
Change-Id: Ib3b78b1c6922539fd5e9238aa017fcdb554aacc8
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Stabilize tst_qfilesystemmodel::rowsInserted() on Windows.

Introduce QTRY_COMPARE to let the sorting finish instead
of a hard-coded time-out.

Task-number: QTBUG-29403
Change-Id: I813bf6bcfe7110f49a5b95db589ed8a3f559db89
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Improve COM error handling in the Windows platform plugin.

Add error strings, print warning if OleInitialize() fails.

Task-number: QTBUG-38398
Change-Id: I37f6b7003fd1cf524ce69d6843891943402b27a1
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>