To ensure that a document frame isn't modified/closed while an
asynchronous scan operation is performed, the frame is blocked
for the lifetime of the shim process.

Change-Id: I7e21c816ca6de8eb0a53498e77eae359985dbf71
Reviewed-on: https://gerrit.libreoffice.org/65727
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>