[motionTwin]  preBuild / postBuild commands

Post Build and Pre Build commands support.

Removal of templates/haxe/DefaultAssetLibrary.hx  Which isn't the one used by the toolchain anymore?  Seems that openfl template is currently used...
So removal and avoid duplicates seems better.

Proposal for an template macro which helps generate DefaultAssetLibrary which all the Reflect things by doing:
type.set ("::id::", AssetType.$$upper(::type::));