Create PidginAddBuddyDialog.

This is a GtkBuilder based version of the old hand built code. Nearly all
functionality. The only thing we dropped was the "auto personize" stuff as
that API is private to gtkblist.c.

Testing Done:
* Added a buddy with and without groups and aliases.
* Verified only accounts that allow adding buddies show up in the account list.
* Verified that hitting enter on the username entry will submit the ok response.
* Couldn't test invite message as no in tree prpls support them.

Reviewed at https://reviews.imfreedom.org/r/1574/