[gaml] introduce user defined types

[lesquoyb]

It's a work in progress

list of goals to attain to merge it to main

• add the syntax to declare in gaml a new type composed of variable and action declarations, and have it recognized by the compiler
• add the newly defined type to the model's list of types
• fill the internal description of the newly defined type with the children variables and actions, this can include other user defined types or the type itself.
• have the possibility to create objects of that type:
  • with an auto-generated constructor-like action
  • optionally through a statement like create
• in gaml being able to access in read/write mode the component variables of an object of a user defined type
• handle serialization/deserialization of those new types
• optionally it would be the occasion to add a spark dataset/dataframe backed type to handle collections of those user defined types, this could bring a lot of benefits like faster processing of large collections of data, easier serialization/deserialization, easy access to complex properties, n-dimensional collections etc.
• The variables of user defined types are visible and browsable in the inspector
• add a parent data_type to the metamodel for common properties (for example the list of attributes)

Problems and bugs so far

• inconsistent naming of type/kind/variables: data, data_type, composed, composed_data
• missing icon for custom types in the model outline
• the default facet is not recognized as the type name (must use the name facet for now
  • in general there are some cases where the default facet name is used and other where it's the name facet that is used: in the outline the default is used for example
• a variable of a user defined type declared in the global cannot be used in the init block
• there's an xtext error the first time we open an existing model from the library