Portal backend request

[iinuwa]

Updates the backend API to act more like others:

• exports an org.freedesktop.impl.portal.Request object with a Close() method to be used for cancellation, instead of a special method on impl.portal.Credential.Ceremony.
  • Note that there are no close signals on impl.portal.Request, as opposed to a portal.Request. The caller should subscribe to the backend request object going away.
• The Initialize() method also now takes handle instead of receiving and returning and returning a handle.
  • This removes the ambiguity between whether the exported path and the given path is correct; it's not always the given handle
  • This also obviates the Start() method, as the caller can subscribe to the signals before sending the call.