Add framework for implementing Linux AudioManager

Source outputs must all be moved to new default source.
Reference topic
https://askubuntu.com/a/72076

* Adds microphone controls on Linux via PulseAudio (PA) for several
points in
https://github.com/OpenVR-Advanced-Settings/OpenVR-AdvancedSettings/issues/43.

It can not be overstated how unsuited the public AudioManager API is for
using the PA API.

It can also not be overstated how undocumented and horrible the PA API
is. The PA way of doing audio is very unlike the Windows way of doing
audio, which the AudioManager is heavily influenced by.

Since I don't have a Linux machine I have not been able to personally
test this. https://github.com/feilen/ has graciously helped with
testing.

PulseAudio is very undocumented and not very widely used for what we
need outside of official utilities. As such there's most likely a better
way of doing things than I've done.
Additional information can be found at
https://github.com/username223/pulseaudio-device-changing.

The "complex" PA API is asynchronous. Since our current AudioManager is
running synchronously and would not work asynchronously (functions are
expected to return valid data on function return) it is necessary to
massage the PA API until it runs synchronously.

This is done with a general workflow of

1. Register a callback function (`pa_context_get_source_info_list()`,
        for example).
2. Run the PA loop through `customPulseLoop()`. This will trigger
any registered callbacks. Sometimes a `void*` is passed through the
callback that can be used for custom data. `successCallback()` uses
this to set the success variable.
3. Run the callback function and do required work.
4. Return to original function, do work or return values.

It is not possible to run a `customPulseLoop()` inside of an already
running loop (inside a callback), as this will lead to a PA assert
error on runtime.

build_scripts/qt/sources.pri

@@ -37,7 +37,6 @@ HEADERS += src/overlaycontroller.h \
     src/utils/Matrix.h \
     src/utils/ChaperoneUtils.h \
     src/quaternion/quaternion.h \
-    src/tabcontrollers/audiomanager/AudioManagerDummy.h \
     src/openvr/openvr_init.h \
     src/openvr/ivrinput_action.h \
     src/openvr/ivrinput_manifest.h \
@@ -86,8 +85,18 @@ unix:!macx {
         SOURCES += src/media_keys/media_keys_dummy.cpp
     }
 
-    SOURCES += src/tabcontrollers/audiomanager/AudioManagerDummy.cpp
-    HEADERS += src/tabcontrollers/audiomanager/AudioManagerDummy.h
+    !noPulse {
+        message(PulseAudio features enabled.)
+        SOURCES += src/tabcontrollers/audiomanager/AudioManagerPulse.cpp
+        HEADERS += src/tabcontrollers/audiomanager/AudioManagerPulse.h \
+            src/tabcontrollers/audiomanager/AudioManagerPulse_internal.h
+        LIBS += -lpulse
+    }
+    else {
+        message(PulseAudio features disabled.)
+        SOURCES += src/tabcontrollers/audiomanager/AudioManagerDummy.cpp
+        HEADERS += src/tabcontrollers/audiomanager/AudioManagerDummy.h
+    }
 }
 
 macx {