[gnome-flashback] Improving documentation

[Lanoxx <lanoxx gmx net>]

Hi everyone,

I have been working on some patches to improve the Panel Library
Reference Documentation in the doc folder of gnome-panel and the API
documentation of libpanel-applet. The current work in progress can be
found in the wip/segeiger/reference-documentation branch of gnome-panel.

I would like to get some feedback and if there is anything you want to
see better documented please let me know and I will see if I can include it.

Additionally to whats already in this branch, I am planning to add the
DBusAppletMigration [1] guide from the Gnome Wiki and add some notes for
GConf to GSettings migration and libpanel-applet 5.0.

Cheers
Sebastian

[1] https://wiki.gnome.org/Initiatives/GnomeGoals/AppletsDbusMigration