Cleanup layer properties

[Desplandis]

Description

This PR introduces several changes to the base Layer and layer-related classes. The main changes includes the migration of LayerUpdateState and LayerUpdateStrategy to typescript, the reorganization of Layer properties for better separation of concerns and the addition of a startup lifecycle method as a first step to cleanup the mess that is View#preprocessLayer. This will not break existing layers within iTowns but could break custom layers inheriting directly from Layer.

Motivation and Context

The motivation for the following changes is as follow:

• Introduction of Layer#startup lifecycle method for separation of concerns, allowing for more explicit initialization of layers. Follow-up: remove View#preprocessLayer and most of the complexity of View#addLayer.
• Migration of LayerUpdateState and LayerUpdateStrategy to typescript (I will personally rebase Further simplify tile subdivision mechanism #2525 on this PR since there is some function paramater changes to chooseNextLevelToFetch).
  Follow-up: migrate TileMesh-related modules to typescript (see Migrate TileMesh-related modules to typescript #2557) / migrate LayerNodeProcessing
• Move ImageryLayers to its own module as I have short-term plan to migrate all layers to typescript (with some type hacks) and have no plan to migrate ImageryLayers (which given the code needs an entire refactoring...).
  Follow-up: migrate Layer modules to typescript
• Move some layers properties to inherited classes for better separation of concerns, facilitate the typechecking of Layer and children.
  • layerUpdateStrategy used only on RasterLayer-derived classes
  • style used only on *3DTilesLayer, FeatureGeometryLayer and ColorLayer
  • mergeFeatures/addLabel used only on FeatureGeometryLayer and ColorLayer
    Follow-up: migrate Layer modules to typescript

Changes considered but not done on this PR:

• Cleanup the mess in View#preprocessLayer
• Moving the caching mechanism in classes where it is effectively used
• Some things that I surely forgot ;)

Screenshots (if appropriate)

[Desplandis]

@AnthonyGlt @mgermerie @HoloTheDrunk If anyone want to review this PR. Especially if you depend on some properties here.