Support custom importers in SDK

[AshleyScirra]

Plugins for custom animations like Spine and Spriter need a way to import bundles of images and data in to Construct. For example a skeletal animation may be formed of a number of images and a data file. Ideally the user can simply drag-drop a zip file with all this data in it, and a plugin handler is called to read all the files and import them as the plugin wants.

This will mean adding the following APIs to the SDK:

• creating new Sprites
• adding animation frames from image files
• customising origin and image points
• setting the animation frame collision polygon
• placing instances in the layout
• creating project files
• configuring containers
• storing the association between the controller plugin and imported sprites. There might need to be UI around this to avoid making it a magic connection from the user's perspective.
• (Need the ability to add actions and conditions with the custom importer sdk #1236) Need the ability to add actions and conditions with the custom importer sdk

There's quite a lot of API surface there but it should all be relatively straightforward, except perhaps for the UI around object associations - I think we would need to invent something new for that.

[AshleyScirra]

There will be an initial version of the custom importer API in r76. Note that real-world importers will need a broad API surface so this may take a few releases to flesh out.

[AshleyScirra]

The first iteration of this API is now available. The entry point is the AddDragDropFileHandler function. Posted a forum update too.

[AshleyScirra]

Mentioned in #1236: Need the ability to add actions and conditions with the custom importer sdk

[AshleyScirra]

Got most of this implemented for the next release. Will update the docs after that release. May be a few more APIs to add to make sure we cover the main use cases but these should be easy to add as we go along. Also there isn't yet anything to store an association but we will see if there are other ways around that depending on what addons need.