Create Xpra-Mic-Source pulseaudio remapped source for Chromium

Applications like Chrome and Chromium do not accept to record from a
monitor device, like "Monitor of Xpra Microphone" device, as reported
at #1221
or werman/noise-suppression-for-voice#29
and so they report that no microphone is connected.

A remapped device resolves this issue by creating a normal (non-monitor)
source device.

The name of the remapped device is **Xpra-Mic-Source** and not
**Xpra-Microphone-Source** because the `get_pulse_device` function from
the `xpra/audio/gstreamer_util.py` file expects to find exactly one
sink device match when looking after the **Xpra-Microphone** name.

xpra/platform/features.py

@@ -39,7 +39,7 @@
 DEFAULT_SSH_COMMAND : str = "ssh -x"
 DEFAULT_PULSEAUDIO_CONFIGURE_COMMANDS : Tuple[Tuple[str, str, str], ...] = (
     ("pactl", "set-default-sink", "Xpra-Speaker"),
-    ("pactl", "set-default-source", "Xpra-Microphone.monitor"),
+    ("pactl", "set-default-source", "Xpra-Mic-Source"),
     )
 
 SOCKET_OPTIONS : Tuple[str, ...] = (

xpra/scripts/config.py

@@ -869,6 +869,7 @@ def get_default_pulseaudio_command() -> List[str]:
         "--exit-idle-time=-1", "--load=module-suspend-on-idle",
         "'--load=module-null-sink sink_name=\"Xpra-Speaker\" sink_properties=device.description=\"Xpra\\ Speaker\"'",
         "'--load=module-null-sink sink_name=\"Xpra-Microphone\" sink_properties=device.description=\"Xpra\\ Microphone\"'",
+        "'--load=module-remap-source source_name=\"Xpra-Mic-Source\" source_properties=device.description=\"Xpra\\ Mic\\ Source\" master=\"Xpra-Microphone.monitor\" channels=1'",
         "'--load=module-native-protocol-unix socket=$XPRA_PULSE_SERVER'",
         "--load=module-dbus-protocol",
         "--load=module-x11-publish",