change object creation and access

The Object class is a virtual proxy which handles every different kind
of objects with the 'itemtype' argument in urls.
generate assets menu entries for each types defined

- ensure menu is generated (actually needs rework since it reload the
  menu on each page visited)
- successful attempt to distinguish plugins' types listed in assets menu
  with some leading icon.
add autoloader specific to the plugin

This autoloader will look for generated classes in
files/_plugins/genericobject
reindenting
remove obsolete code
use of class self-method to get defined types

... instead of having a duplicate method that does the same.
Fix menu names capitalization
reindent
fix right on creation check
move types creation in config menu
remove right check

This is normally done later in the showForm methods of a model.
add a script to regenerate the plugin files

This is helpfull while developping and debugging in order to avoid to
uninstall/install the plugin.