KDE4: evaluate frameWidth in the Qt thread

When opening a document via Java UNO without a running LO instance, one
gets the following Qt error messages:

CE>  QObject: Cannot create children for a parent that is in a different thread.
CE>  (Parent is Oxygen::WidgetStateEngine(0x8deb878), parent's thread is QThread(0x8d6cf70), current thread is QThread(0xa8fa7fc8)
CE>  QObject::installEventFilter(): Cannot filter events for objects in a different thread.
CE>  QObject::installEventFilter(): Cannot filter events for objects in a different thread.
CE>  QObject::installEventFilter(): Cannot filter events for objects in a different thread.

This happens, because the Java UNO call is processed in the first /
Qt thread while document loading happens in a second thread.

Document loading actually just calls getNativeControlRegion, which
should not involve any drawing. But the KDE4 backend does some style
processing to get the correct frame width (QWidget::ensurePolished(),
which uses GUI based events and need to be processed in the Qt
thread.

(cherry picked from commit 7163d64b90ac4d4259b1d0379cfca348dd30601c)

Change-Id: I8a0bf7c22e517bbe163bff1465f88ae422fd8767
Reviewed-on: https://gerrit.libreoffice.org/8702
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>