Move PurpleProxyInfo from a boxed type to a GObject

As well as:

* Fix the namespace of the PurpleProxyType members.
* Remove the GNOME proxy detection code.
* Remove the Windows proxy detection code.

Testing Done:
Created an account and messed around with the proxy settings and verified they were saved and restored.

Reviewed at https://reviews.imfreedom.org/r/1368/