checkin: capability to parse checkin responses into something sane

Parse checkin response messages into a struct checkin. We
don't really really care about the structure, we just pull
in all human-readable messages for now.

It's not hooked into anything yet either, but there's a unit
test with a genuine checkin response to validate parsing.
checkin: include messages from response->notifications

We still don't care about providing checkin data in any structured
manner, but the notifications blob in the response contains some
nice messages too.

Include them in our information.
checkin: show checkin result info in a dialog

Render the messages collected in struct checkin in a dialog after
checkin.
test: only run the gtk dialog test if the user wants to

We don't have the automated bits to test gtk dialogs yet, and it's
a bit offensive starting to throw windows at the user when running
tests.

So hide the checkin result tests from normal operation and only
run them if '--interactive' is passed to the test run script, via
$(TEST_ARGS) for example
checkin: parse venue name into struct checkin
checkin: include the newly-parsed venue data in the checkin info dialog