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