Add button to apply Lightmap slots to selected items

[mdwigley]

Detailed Description

Create a button to set all selected PrefabLightmapData components to activate the slot with the slot name in the "Lightmap Slot Name" input field.

This request should also update the documentation to include a description of the new button and update the tool image with it visible.

Context

Enabling previously baked PrefabLightmapData components can be difficult, especially in large or complex scenes. This difficulty can be made trivial by having a dedicated button for this action.

Possible Implementation

Insert a new button named "Apply" between the "Clear" and "Bake" buttons, which on button press, sets all selected items to the slot name in the "Lightmap Slot Name" field.