pidgin/pidgin

Add purple_menu_populate_dynamic_targets to dynamically update GMenu's

This is going to be used in the GMenu's for plugins, protocols, accounts, etc where there are dynamic items and we need to be able to set a target for them dynamically.

Testing Done:
Ran the new unit tests.

Reviewed at https://reviews.imfreedom.org/r/1356/
Björn Voigt <bjoernv@arcor.de> Björn Voigt <bjoern@cs.tu-berlin.de>
Björn Voigt <bjoernv@arcor.de> Bjoern Voigt <bjoern@cs.tu-berlin.de>
David Woodhouse <dwmw2@infradead.org> <David.Woodhouse@intel.com>
David Woodhouse <dwmw2@infradead.org> <dwmw@amazon.co.uk>
David Woodhouse <dwmw2@infradead.org> dwmw2 <dwmw2@infradead.org>
# not sure how to make this one work because this doesn't do it..
dequis <dx@dxzone.com.ar> dequis <dx@dxzone.com.ar
dequis <dx@dxzone.com.ar> dx <dx@dxzone.com.ar>
Eion Robb <eion@robbmob.com> Eion Robb <eionrobb@gmail.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com> Elliott Sales de Andrade <qulogic@pidgin.im>
Elliott Sales de Andrade <quantum.analyst@gmail.com> Elliott S <quantum.analyst@gmail.com>
Ethan Blanton <elb@pidgin.im> Ethan Blanton <elb@fiji-systems.com>
Ethan Blanton <elb@pidgin.im> Ethan Blanton <elb@kb8ojh.net>
Gary Kramlich <grim@reaperworld.com> <grim@pidgin.im>
Jerin Philip <monu1618@gmail.com> Jerin Philip <jerin.philip@students.iiit.ac.in>
Jonathan Champ <jrchamp@ncsu.edu> Jonathan Champ <royanee@users.sourceforge.net>
Jorge Villaseñor <salinasv@pidgin.im> Jorge Villaseñor <masca@cpw.pidgin.im>
Mark Doliner <mark@kingant.net> Mark Doliner <markdoliner@pidgin.im>
Matěj Cepl <mcepl@cepl.eu> Matěj Cepl <mcepl@redhat.com>
Michael McConville <mmcconville@mykolab.com> Michael McConville <mmcco@mykolab.com>
Michael McConville <mmcconville@mykolab.com> mmcconville@mykolab.com
Mike Ruprecht <cmaiku@gmail.com> Michael Ruprecht <maiku@pidgin.im>
Richard Laager <rlaager@wiktel.com> Richard Laager <rlaager@pidgin.im>
Robert Vehse <robertvehse@fastmail.fm> Robbie <robertvehse@fastmail.fm>
Shalom Craimer <scraimer@gmail.com> Shalom Craimer <s.transifex2015@craimer.org>
Tomasz Wasilczyk <twasilczyk@pidgin.im> Tomasz Wasilczyk <tomkiewicz@cpw.pidgin.im>
Yngve Spjeld Landro <l10n@landro.net> Yngve Spjeld Landro <nynorsk@strilen.net>