Unit selector for object layers

**Is your feature request related to a problem? Please describe.**
I would like to have an object layer with objects positioned tile-relative, currently all objects are world relative.
Is there a way to do this? read the whole "Working with Objects" docs and doesn't seem so.

I'm making a tile-based game and using an object layer to define world events. Events are positioned per tile, so objects with a world coordinate are inconvenient.

**Describe the solution you'd like**
An option to make the object layer tile-relative, and they render correctly while editing.

**Describe alternatives you've considered**
Doing a post-process of the generated tilemap file to make the objects tile-relative.
This doesn't help while editing tho

Start by rereading the “Working with Objects” documentation and tracing the object-layer editing and rendering paths. Confirm how object coordinates are currently interpreted, then define the tile-relative behavior for both editing and rendering; completion should support the requested unit selector without requiring post-processing of generated tilemaps.

Written by the indexing model from the issue text.