Add signal sending capability

Georg Chini requested to merge gchini/pulseaudio:signals into master

As a counterpart to the messaging API the patches enable applications to receive signals from pulseaudio. The mechanism is similar to the subscription API, the client can subscribe to signals and set a callback that will be called, when a signal is issued. A 64 bit subscription mask allows to distinguish between different types of signals. The types of signals are not yet defined.

In comparison to the subscription API, signals are more flexible and allow to send detailed information. The callback function receives a sender string, a signal name and a signal parameter list in the messaging API format.

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