Add support for multi-tile objects in Object Layers

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### Is your feature request related to a problem? Please describe.

Currently, when creating a Tile Object in the Object Layer, the system is strictly limited to a single tile per object. This forces users to handle multi-tile structures (like a larger building or a complex pillar) as a collection of individual objects, which makes bulk manipulation and logical grouping difficult.

_I've seen this same headache in so many games over the years, everyone ends up hacking together clunky workarounds like templates or complex flood-filling logic in their engine, so having this handled directly in Tiled would be a huge game-changer._

### Describe the solution you'd like.

The user should be able to select a group of tiles from a tileset and insert them as a single object instance in the Object Layer, similar to how the current "Tile Object" works, but capable of holding multiple tiles. This object would internally store the relative positions of all selected tiles, allowing them to be moved, rotated, and scaled as a single component.

### Describe alternatives you've considered.

I have considered using Templates or engine-side logic (like flood-filling), but these are clunky, hard to maintain, and often lead to issues with Y-sorting and depth management in the game engine. Having this feature directly in Tiled would simplify both the editor workflow and the game engine integration significantly.

No file or test is named. Start by tracing how the existing Tile Object is created and represented in Object Layers, then compare that with tileset selection and transform behavior. Done means a selected group can be stored as one object with relative tile positions and moved, rotated, and scaled together.

Written by the indexing model from the issue text.