Web Inspector: asynchronously initialize saved breakpoints.

This ensures that all managers learn about restored breakpoints
in the same order, regardless of manager initialization order.
Probes: fix a typo that broke additional sidebar panels from appearing.
Web Inspector: convert breakpoints to have a tri-state 'mode'.

The mode can be 'enabled', 'autocontinue', or 'disabled'. The UI
reflects these different modes with opacity settings. Clicking the
breakpoint markers will cycle between these modes.

The auto-continue mode only makes sense if there are actions
to evaluate, otherwise there is no effect from hitting. So, cycling
will not go to the auto-continue state unless the breakpoint has
actions.

Lastly, the breakpoint edit popover hides auto-continue when there
are no actions added for the breakpoint.
Probes: remove dead code that won't be coming back.
Probes: remove the Probes domain from the Web Inspector.
Probes: add new 'probe' breakpoint action type, which currently does nothing.
Probes: obtain and pass breakpoint action identifier to backend.
Probes: collect probe samples and send them through to the frontend.
Probes: Reimplement the probes model in the frontend.

ProbeGroups have been renamed to ProbeSet. Each probe and probe set is
associated with a Breakpoint instance, which may or may not be
resolved in the backend. Probes cannot be enabled or disabled
themselves.
Probes: make the probe details sidebar eligible for display again.
Probes: fix the button to clear an entire probe set.
Web Inspector: remove breakpoint actions prior to firing breakpoint removed event.
Probes: show all probe sets in the sidebar for the currently-open file.