fix(multiselect): complete refactor of multiselect module

inputEx.MultiSelectField inherited from inputEx.SelectField and that was a bad design
cause value come from the DDList, not from the select

So now, the inputEx.MultiSelectField inherit from inputEx.DDListField for a more
logical API
fix(multiselect): update clear and constructor methods after Maxime comments
Merge pull request #110 from neyric/multiselect-refactor

fix(multiselect): complete refactor of multiselect module