package/wpewebkit: add option to enable sandboxing support

package/wpewebkit: add option to enable sandboxing support

Add an option to enable WebKit's sandbox, which uses kernel
namespaces to isolate the processes used for Web content rendering
(WebKitWebProcess) and network/disk access (WebKitNetworkProcess).

The reason to have an option is that it needs additional dependencies
(bubblewrap, xdg-dbus-proxy, libseccomp), and that some users may
choose to deploy alternative solution (for example: putting all
of WebKit inside its own container, using systemd-nspawn or the
like).

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
[Peter: select libseccomp]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>