As part of this change an API is introduced to add custom visual elements (inlays) into editor, not reflected in document text.
At the moment only passive inline elements with height equal to line height are supported.

Current implementation is draft in following aspects:
- It is not optimized for memory usage - keys are based on raw Strings, this is inefficient
- No smart caching of results
- Indices (equations) are not persistent and are in memory for now