PWF js locales

* Added translation js script
PWF js locales

* Fixed pwf js locales init
PWF js icons

* Added pwf js module for icons
Locales

* Changed some locales to be more intuitive
Forms, selected option

* Fixed comparing element object value to int
Database model, belongs_to ints

* Database belongs_to relations accepts integers
Database model, belongs_to

* Belongs_to relation now accepts empty values as NULL
Attr model, attr_exists

* Added protection for empty values to Model\Attr::attr_exists
Database model, has_many

* Added better validation inside database models' has_many getter