Default-Initialize new AutoTile Items in Tilesets

Summary

Right now, every AutoTile in a Tileset has to be configured manually, even if its AutoTiles are laid out regularly in a grid. When creating a new AutoTile, it could copy and offset the previous items config to avoid repetitive work.

Analysis

• Determine the previous AutoTile's position and size on the Tileset in tiles, offset by its own size to determine the new AutoTile's rect.
• The offset should work left-to-right, with line breaks where the end is reached.
• Within the rect, copy IsAutoTile and Neighbours from the previous tile to the new one.
• AutoTileTilesetEditorMode.AddLayer is probably a good place to start integrating this.