Standardize names and arguments for Composite's mergeReqs and mergeModel

[fjvallarino]

These changes were made for consistency and to allow the same use cases as with regular widgets.

• Renamed:
  • customModelBuilder -> compositeMergeModel
  • CompositeCustomModelBuilder -> MergeModelHandler.
• Added
  • compositeMergeEvents
• Changed
  • compositeMergeModel (previously customModelBuilder) now receives WidgetEnv as its first parameter.
  • compositeMergeReqs now receives parentModel and oldModel too.

Note: compositeMergeModel does not receive the node and oldNode because node is created using the model, and oldNode may not exist when invoked during onInit.