Again, mdds starts container search always from the beginning, so
iterating over it is quadratic. Making a large selection triggers
VclQt5Clipboard::setContents(), which calls this.

Change-Id: Ic027ff7c5507b41d42b9561cd099e01b50055adb
Reviewed-on: https://gerrit.libreoffice.org/72367
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>