decode: Prepare to rework text output for protocol decoder annotations

decode: Prepare to rework text output for protocol decoder annotations

When the current implementation receives a protocol decoder annotation,
one out of several code paths prints one of several different variations
of text layout. The current implementation exclusively checks the active
log level to determine the layout variant.

Prepare to rework that text layout logic. Separate the determination of
which fields to display from the logic which does emit those fields in a
specific format.

This commit does not yet adjust the print statements. It is supposed to
verify that the "which fields to display?" logic results in identical
behaviour to the previous implementation.