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/
syntax: glob
.*.swp
.DS_Store
.flatpak-builder
*.pyc
*~
\#*\#
syntax: regexp
^build(.+)?\/
win32-install-dir(\.release)?
po\/pidgin3\.pot$
^subprojects\/.+\/