Partially translate to french
(transl) Add LangManager to handle translation
(transl) Improve french localization
(LangManager) Transform into singleton
(LangManager) Add a method to get list of available translations
Cosmetic changes
(settings) Add a new observer (ComboBox)
(settings) Add area to select language
(settings) Implements values for language combobox
(main) Destroy the LangManager instance before closing program
(settings) Store language in a better way, using UserDataRole
(LangManager) Add a slot to update translation from QSettings
Cosmetic changes
(misc) Update translation at some points

1. When application starts
2. After accepting settings dialog
(LangManager) Do not forget to clan pointers when deleting
(LangManager) Add Doxygen for the class
(docs) Improve Doxygen support

More improvements needed to have a nicer documentation
Merge branch 'translation_implementation'
(i18n) Update french translation
(i18n) Delete useless english translation