Add PurpleAccount:connected and PurpleAccount:disconnected signals

This will be propagated by PurpleAccountManager at a later time, but were
implemented separately.

Testing Done:
Temporarily added `g_warnings` where the signal emissions where to verify they were being emitted.

I didn't add unit tests for this as there's a lot of boiler plate for that which will hopefully be changing soon.

Reviewed at https://reviews.imfreedom.org/r/2439/