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>

src/core/meta-context-private.h

@@ -18,6 +18,7 @@
 
 #pragma once
 
+#include "core/meta-debug-control.h"
 #include "core/meta-private-enums.h"
 #include "core/meta-service-channel.h"
 #include "core/util-private.h"
@@ -84,3 +85,5 @@ meta_context_get_profiler (MetaContext *context);
 void meta_context_set_trace_file (MetaContext *context,
                                   const char  *trace_file);
 #endif
+
+MetaDebugControl * meta_context_get_debug_control (MetaContext *context);