Make PurpleRequestFieldAccount work like a GtkCustomFilter

This makes the filtering better for introspection.

Also, fixed an incorrect type check.

Testing Done:
Enabled Demo and XMPP accounts, then tried the I'dle Mak'er plugin and confirmed that only XMPP showed in its "Set Account Idle Time" action request.

Bugs closed: PIDGIN-17780

Reviewed at https://reviews.imfreedom.org/r/2442/