Roster: Fixed crash on invalid iter.
GSettings: Converted more keys to enums. Done cleanups.
GSettings: Converted keys to enum.

A few changes:
- gmmenuaddon has been enhanced to support enums for radio entries
- generic radio widgets <-> string/int gsettings mappings have been
  added
- the main and call windows have been ported to the new scheme

I reorganised the file structure during this work.
Makefile: Remove ekiga.schemas which does not exist anymore.
GSettings: Added GConf to GSettings migration file.
GSettings: Added range specifications.

This allows preventing sane editors to put wrong configuration values.