This patch adds WebView::ViewImplementation with all the
`notify_server_did_this_or_that()` functions from OOPWV.
This will allow us to share code between different web views, paving the
way for a Qt widget in Ladybird that can talk to a WebContent process.
To achieve this goal:
- The Browser unveils "/tmp/portal/filesystemaccess"
- Pass the page through LoadRequest => ResourceLoader
- ResourceLoader requests a file to the FileSystemAccessServer via IPC
- OutOfProcessWebView handles it and sends a file descriptor back to
the Page.
Also moves WebContentClient and the references to the generated IPC
descriptions, since they are all components of OutOfProcessWebView.
This patch has no functional changes.
