Create a PidginProxyOptions widget

This replaces the old page in the account editor and uses a PurpleProxyInfo
object to for editing.

Testing Done:
Verified it populated saved values on an existing account, as well as saved values on existing account.
Also verified that you can create a new account with proxy options and that they're saved.

Reviewed at https://reviews.imfreedom.org/r/1377/