Add option to replace current open scene in Find Usages

[citizenmatt]

The Unity Find Usages window lists all usages of an item across all scenes. When selecting a usage from another scene, the Unity editor window will prompt for confirmation before opening the new scene. The new scene is then opened additively, meaning the current scene is still open, too. We should add the ability to replace the current scene, rather than loading multiple scenes at once.

Change the dialog to read:

Do you want to close the current scene and open scene "%s"?

| Open Scene | | Open Scene Additive | | Cancel |

"Open Scene Additive" is the same text as used in the context menu of scene assets.

[citizenmatt]

See tweeted feature request 😄.