types: fix untyped model when using ElementView/LinkView

[michaelspiss]

Description

Instead of passing a generic Element/Link type to CellViewGeneric this provides an optional generic to pass the model type. It would be used like the following:

interface CustomElementAttributes extends dia.Element.Attributes {
  custom_attribute: string,
}

class CustomElement extends dia.Element<CustomElementAttributes> {
  // ...
}

class CustomElementView extends dia.ElementView<CustomElement> {
  // ...
}

Default behaviour is the current behaviour. The change is fully backwards compatible.

Motivation and Context

• this.model.get(...) is checked against attributes of the model
  • return value is typed instead of any
  • language server knows about possible keys
• No longer throws errors ("... implicitely has any type") when used with typescript
  • Removes manual overhead of hard coding types back in after using get()

[kumilingus]

Just this one undocumented method needs to be updated.

Good work!