Use standard GTK tooltip in the buddy list.

Instead of drawing this all ourselves, drop it all in a `GtkGrid`. This handles all the padding and alignment for us, which is way more consistent, too.

Note, old code has not been removed because it's used in conversations.

Testing Done:
Hover over buddy list, try with keyboard `Ctrl+F1`.

Reviewed at https://reviews.imfreedom.org/r/754/