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/
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