When a child process exits, the SIGCHLD handler was destroying the
Process object synchronously within the signal dispatch context. This
triggered Process::~Process() which called m_connection->shutdown(),
attempting to join the IPC IO thread.
Joining threads from within signal handler context can cause deadlocks,
as observed when WebContent crashes during test-web runs.
Fix this by deferring the on_process_exited callback using
Core::deferred_invoke(), ensuring the Process destruction happens in
normal event loop context.
The intent is that this will replace the separate Task Manager window.
This will allow us to more easily add features such as actual process
management, better rendering of the process table, etc. Included in this
page is the ability to sort table rows.
This also lays the ground work for more internal `about` pages, such as
about:config.
