This patch deals with what is likely the most common use case:
Filtering specific messages at a particular location. The current
granularity is essentially per-file (and per-function, where possible),
which seems more useful than taking line numbers into
account, as that would not be robust with regards to code changes elsewhere
in the file. We can fine-tune this if the need arises.

Change-Id: I4e9b2671fa199339cc3b995953d072b840cd3205
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>