show according message when triggers are processed

[Jonas]

Processing triggers at the end of an update may take some time. Especially mandb needs some time to update its index.
The update manager displays that the last package was installed, but does not reflect that triggers are being processed.
This may be confusing to users or raise questions as to why no progress is being made.
I would appreciate if the message under the progress bar would show that the mandb index is being updated or that triggers are being processed to clarify what is happening.

Thanks for making updates more accessible.