Placeholder invite-accept UI

This is a lightweight placeholder so calendar functionality can be
tested.  Simply presents a message about the invitation, and a set of
yes/maybe/no buttons to click.

The UI is shown whenever the message appears to contain an invite.

There are many elements left to be done here:
TODO: response code (EAS protocol) doesn't seem to work
TODO: use real assets & design
TODO: provide a click-link into calendar event
TODO: show calendar icon in messagelist too
TODO: (if possible) persist user's response in button state?
1 file changed