Mechanism to provide users with "Info" messages (as opposed to "Warnings")

[lindsay-stevens]

There are cases (e.g. here in 370 and 571) where pyxform may detect potential usability or correctness issues with a XLSForm, but those issues don't violate the XLSForms/XForms spec or cause bugs in a way that a warning would be warranted.

From a user's point of view, perhaps there could be an optional command line flag to --show-info-messages or similar. Internally it may make sense to switch (admittedly, a big switch) from passing around a warnings list, to using a customised logging handler (relevant to #330, #401), so that messages can be added like log.warning(msg) or log.info(msg) rather than passing around 2 lists.

The scope for this ticket would be to add the mechanism, and at least one info message that uses that mechanism.