Add debug controller

The debug controller can optionally, when passing --debug-control,
enable manipulating debug state, so far enabling/disabling HDR, via
D-Bus.

It's always created, in order to have a place to store debug state and
emit signals etc when it changes, but so far, it doesn't have its own
state it tracks, it just mirrors that of the monitor manager.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3432>

tools/debug-control.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+
+import argparse
+import dbus
+
+NAME = 'org.gnome.Mutter.DebugControl'
+INTERFACE = 'org.gnome.Mutter.DebugControl'
+OBJECT_PATH = '/org/gnome/Mutter/DebugControl'
+
+PROPS_IFACE = 'org.freedesktop.DBus.Properties'
+
+def bool_to_string(value):
+    if value:
+        return "true"
+    else:
+        return "false"
+
+def get_debug_control():
+    bus = dbus.SessionBus()
+    return bus.get_object(NAME, OBJECT_PATH)
+
+def status():
+    debug_control = get_debug_control()
+    props = debug_control.GetAll(INTERFACE, dbus_interface=PROPS_IFACE)
+    for prop in props:
+        print(f"{prop}: {bool_to_string(props[prop])}")
+
+def enable(prop):
+    debug_control = get_debug_control()
+    debug_control.Set(INTERFACE, prop, dbus.Boolean(True, variant_level=1),
+                      dbus_interface=PROPS_IFACE)
+
+def disable(prop):
+    debug_control = get_debug_control()
+    debug_control.Set(INTERFACE, prop, dbus.Boolean(False, variant_level=1),
+                      dbus_interface=PROPS_IFACE)
+
+def toggle(prop):
+    debug_control = get_debug_control()
+
+    value = debug_control.Get(INTERFACE, prop, dbus_interface=PROPS_IFACE)
+    debug_control.Set(INTERFACE, prop, dbus.Boolean(not value, variant_level=1),
+                      dbus_interface=PROPS_IFACE)
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(description='Get and set debug state')
+
+    parser.add_argument('--status', action='store_true')
+    parser.add_argument('--enable', metavar='PROPERTY', type=str, nargs='?')
+    parser.add_argument('--disable', metavar='PROPERTY', type=str, nargs='?')
+    parser.add_argument('--toggle', metavar='PROPERTY', type=str, nargs='?')
+
+    args = parser.parse_args()
+    if args.status:
+        status()
+    elif args.enable:
+        enable(args.enable)
+    elif args.disable:
+        disable(args.disable)
+    elif args.toggle:
+        toggle(args.toggle)
+    else:
+        parser.print_usage()