pidgin/pidgin

Remove PidginWindow as it wasn't used and clean up some old PidginWindow -> PidginConvWindow that were previous missed.

Testing Done:
Compiled, built the pidgin docs, and pidgin.pot.

Reviewed at https://reviews.imfreedom.org/r/826/
subdir('libpurple')
subdir('protocols')
if ENABLE_GTK
subdir('pidgin')
else
pidgin_doc = []
endif
if enable_consoleui
subdir('finch')
else
finch_doc = []
endif
alias_target('doc', libpurple_doc, protocols_doc, pidgin_doc, finch_doc)