There is a lot needed all at once to actually inspect a tab's DOM tree.
It begins with requesting a "watcher" from a TabActor. It seems there
can be many types of watchers, but here we implement the "frame" watcher
only. The watcher creates an "inspector", which in turn creates a
"walker", which is the actor ultimately responsible for serializing and
inspecting the DOM tree.
In between all that, the DevTools client will send a handful of other
informational requests. If we do not reply to these, the client will not
move forward with the walker. For example, the CSSPropertiesActor will
be asked for a list of all known CSS properties.
Previously, we could connect to our DevTools server from Firefox, but
could not see any information on Ladybird's opened tabs. This implements
enough of the protocol to see a tab list, but we cannot yet inspect the
tabs.
To aid with debugging web page issues in Ladybird without needing to
implement a fully fledged inspector, we can implement the Firefox
DevTools protocol and use their DevTools. The protocol is described
here:
https://firefox-source-docs.mozilla.org/devtools/backend/protocol.html
This commit contains just enough to connect to Ladybird from a DevTools
client.
