Added a Sprite.tileManagementPlugin property for plugins that want to
replace the standard tilemap/tileset interface. This includes a new
external file field in .aseprite files to specify that the sprite
tiles are controlled by a specific plugin.
Once this property is set, the standard tilemap/tileset modes
selectors will disappear and the only way to make then available will
be setting this property to nil/empty string again.
Fix https://github.com/aseprite/Attachment-System/issues/21
Don't use GitHub tables for the spec, they don't work for this kind of
documents. I've restored the old layout using just plain text to
describe chunks and fields.
Changes:
* Added SeparatorInView() to add separators inside a ListBox or ComboBox.
* Added BlendModeItem() to represent each item in the blending modes
ComboBox.
* Now a ComboBox can contain any kind of widgets as children (not just
ListItem). This is required to add separators in the blending modes
ComboBox.
Feature request:
https://community.aseprite.org/t/additive-blending-feature/121
