Fix and enhance creation/destruction of tabs

Websites can now open or close tabs/windows if requested by the user.

- We must not destroy the window in its destroy handler.
- WebKit's "new-window-policy-decision-requested" is totally irrelevant.
- Handle WebKit's "create-web-view" by allocating a new window and an
  "empty" WebView.
- Handle WebKit's "close-web-view" by destroying the window, which will
  call client_destroy(), which will do exactly the same as closing the
  window manually.