fix(calls): create dummy source for invalidated callback

[anthonybilinski]

Fixes mute friend and group calls when mic is switched to disabled then back to a valid source. Now ToxCall can register to the DummySource, and re-register when the source is changed, like when two normal sources are used.

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This change is 

[sudden6]

Reviewed 5 of 8 files at r1.
Reviewable status: 0 of 1 LGTMs obtained

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src/audio/backend/openal.h, line 152 at r1 (raw file):

    std::unordered_set<AlSink*> sinks;
    std::unordered_set<AlSink*> soundSinks;
    std::unordered_set<IAudioSource*> sources;

Mixing ALSource and DummySource here is probably a bad idea, because AlSource actually represents resources in the audio backend and needs to be destroyed differently

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src/audio/backend/openal.cpp, line 329 at r1 (raw file):

    std::unique_ptr<IAudioSource> source;
    if (!Settings::getInstance().getAudioInDevEnabled()) {
        source = std::unique_ptr<IAudioSource>(new DummySource{*this});

Bad dependency on the Settings singleton

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src/audio/backend/openal.cpp, line 366 at r1 (raw file):

    qDebug() << "Unsubscribed from audio input device [" << sources.size() << "subscriptions left ]";

    if (sources.empty()) {

Having dummy sources in sources will keep the audio device open even though there are no real sources

[sudden6]

what do you think about IAudioControl handling dummy source/sink logic, and dispatching to a virtual private function that OpenAl implements for creating /real/ audio sources and audio sinks?

So you want to turn the IAudioControl interface into a full class, right? ATM I don't see any drawbacks from this approach.