Add audio IPC options

AudioService: add a few functions to AudioService
IPCManager: Add 4 Audio IPC calls
README: Add information about new IPC calls

Modules/IPC/IPCManager.qml

@@ -99,6 +99,24 @@ Item {
     }
   }
 
+  IpcHandler {
+    target: "volume"
+    function increase() {
+      AudioService.increaseVolume()
+    }
+    function decrease() {
+      AudioService.decreaseVolume()
+    }
+    function muteOutput() {
+      AudioService.setMuted(!AudioService.muted)
+    }
+    function muteInput() {
+      if (AudioService.source?.ready && AudioService.source?.audio) {
+        AudioService.source.audio.muted = !AudioService.source.audio.muted
+      }
+    }
+  }
+
   IpcHandler {
     target: "powerPanel"
     function toggle() {