use namespace in jelix

[laurentj]

we could use namespaces for all jelix components.

Proposition of namespaces :

• jelix: global namespace
• jelix\db : namespace of jDb
• jelix\dao : namespace of jDao
• jelix\core : namespace of files of the core
• etc. for all components inside lib/jelix/. the idea is to use jelix:name_of_component, where name_of_component is the name of the directory inside lib/jelix, so the autoloader could load easily the component.

For modules, we could use jelixmodule\modulename\a_class to load a class of a module with the autoloader.

[laurentj]

(In #463) For Jelix 1.5, we have been done all that was possible without breaking everything.