Use standard GTK tooltips in the conversation.

Testing Done:
Compiled only. I'm not able to trigger this tooltip in Pidgin 2 on the user list, so I don't know how to test here. I can trigger a tooltip on the info pane in Pidgin 2, but that hasn't been implemented for the new Pidgin 3 info pane widget yet.

Reviewed at https://reviews.imfreedom.org/r/755/