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/
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