add "ringing tone"

[r10s]

on an outgoing call, currently, after "connecting" successfully, the app shows the information "ringing" on the screen, which leads to the ring sound at the other end.

however, the caller does not have any audible sign.

during ringing, we should therefore play a "ringing tone" (not to be confused with "ringtone"), this is what the user knows from the ux from phones, landline, but also outgoing calls with apps on iOS eg.

the "ringing tone" as the following advantages:

• you get an audible hint when the other side actually starts playing ringtone. this is esp. important when connection phase takes longer and one is not looking at the display all the time

• you get to know if your loudspeaker is working - if the "ringing tone" is missing tho connection has established, smth is wrong and you can maybe tune up volume even before call starts

• mimic known ux

for the tone itself, just reusing the ringtone is confusing. using the standard tone suggested at https://en.wikipedia.org/wiki/Ringing_tone seems to be a good choice

[WofWca]

Web Audio API can be used by this perhaps, instead of an audio file.

[r10s]

we are playing & looping an audio file already for ringtone, so at a first glance, it seems easier to do the same. EDIT: this assumption was wrong, the ringtone is played by desktop at deltachat/deltachat-desktop#5489

still, the size of a few second audio file can probably be neglected - so, whatever is easier and less code and complexity :)