notify adding reactions

[r10s]

this PR adds an event for reactions received for one's own messages.

this will allow UIs to add notification for these reactions.

Screenshots at deltachat/deltachat-ios#2331 and deltachat/deltachat-android#3377

[link2xt]

I thought Android relies on getFreshMsgs to return what should currently be in the notifications, but apparently it is not the case.

Wonder if we should deprecate getFreshMsgs and replace it with getFreshMsgCount(0) or something like this. Counting messages should be cheaper than passing arrays around.

[r10s]

at least on android, notifications are driven by DC_EVENT_INCOMING_MSG - and with this PR also DC_EVENT_INCOMING_REACTION.
i did not look further into getFreshMsg() when i realised that the reactions have no ID :)

but yes, it seems at least on android only getFreshMsg().length is used - having that as getFreshMsgsCount(0) would be nicely fitting to other places where the API uses this pattern. i did not check other OS yet, however

[r10s]

thanks a lot, @iequidoo, for review and the many tips! ❤️