Please add a "Solo" layer option

It would be incredible useful if tiled would support solo-layering (Make ony single layer editable only as needed - all others but readable). An icon next to the checkbox with a dot indicates when a layer is "solo". Double click on layer will set/toggle solo - like in photoshop.

Start by locating Tiled's layer list/editor entry point and the existing checkbox and double-click handling. Confirm how editability and layer indicators are represented, then define done as one layer being solo-editable, other layers remaining readable, and a visible dot showing the solo state.

Written by the indexing model from the issue text.