already before,
we did _not_ sent updates to contact requests or other read-only-chats.
however, we _did_ modify the local database,
so that getAllUpdates() returns an update that was not sent out to other peers.
this is fixed by checking can_send() soon.
that way, all peers have the same state
also for contact requests or other read-only-chats
and webxdc in contact requests can be opened as usual.
further ui improvements may be needed for contact requests
(maybe allow the webxdc to know about that state or
maybe show a warning in the ui somewhere),
however, this is not part of this pr.
* let sending invalid webxdc fail
invalid webxdc can still be send as Viewtype::File, however
(maybe one want to discuss errors or so ;)
* clarify the supported zip compression methods
the summary can be modified by the apps using
`sendUpdate({summary: "foo", payload: ...})`
the summary is updated when there is no newer update
and chat will be informed by the change as usual by
`DC_EVENT_MSGS_CHANGED`.
additional parameters will be `summary` and `msg` (for an info-message)
right now, and maybe more in the future (`aggregated`, `fast`, `to` ...),
so adding just more parameters easily gets wild.
also, this makes adaptions easier as no ffi needs to be adapted
when we add more parameters.
finally, sending is in-sync with receiving, database and wire now,
one receives similar objects as one sends,
which also looks like the right thing :)
for now, `sendUpdate()` also allows to use the old, unwrapped payload
to not immediately break existing `.xdc`, however, that will be removed soon,
probably before the first release.
