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/
For information on translating Pidgin, libpurple, and Finch, please see
our tranlsation page at https://pidgin.im/development/i18n/