pidgin/pidgin

use a globally unique id when creating new Pidgin::DisplayItem's

Use a globally unique id when creating new `Pidgin::DisplayItem`s as the doc demands and as discussed on [discourse](https://discourse.imfreedom.org/t/how-is-pidgin-id-supposed-to-be-unique/134/2)

Testing Done:
Ran Pidgin and opened a conversation.

Reviewed at https://reviews.imfreedom.org/r/2980/
if get_option('doc')
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)
endif