clang-tidy: Avoid warnings for calling virtual methods from ctor

clang-tidy: Avoid warnings for calling virtual methods from ctor

MessageFilter and its derivatives call their idString() method
during construction in order to access their settings. This is fine,
but causes (legitimate) warnings in static analysis.

Make the scope explicit when calling idString() to avoid the warning,
and make it clear to readers that we're not interested in the derived
version.