connect `request-queued` signal conditionnally

- add a new function: vb_update_signals()
  it manage signal connection/disconnection

  currently, only `request-queued` is managed. It is connected when
  autocmd use event AU_REQUEST_QUEUED or when setting
  "content-security-policy" is in use.

- remove the unconditionnally connection of `request-queued`

- add a new flag in setting `FLAG_SIGNAL` that inform setting_set_value
  to refresh signals by calling vb_update_signals()

- manage "content-security-policy" setting with `FLAG_SIGNAL`

- refresh signals in autocmd_add, at adding or removing. This permit to
  change the connection/disconnection state of `request-queued` when
  AU_REQUEST_QUEUED is in use or not.