Add DebugControl D-Bus API and hook it up to enabling experimental HDR

Testing various things with mutter is tricky when one wants to change configuration without restarting. An example of this is HDR, where one really wants to compare how it looks enabled vs disabled.

To address this, add a org.gnome.Mutter.DebugControl D-Bus API that gets enabled by passing --debug-control. It's intended to be a place to expose and potentially store debug. For now it just pokes the monitor manager's existing debug knob.

In another branch I add another "debug control" (blending mode), and it's meant to make it easy to add more, even temporary while hacking on some branch.

There is also a env var added for enabling HDR, which I used first, but I left it in. Can remove if need be.