UI: Add _CHANGING frontend events

Adds the following events:
- `OBS_FRONTEND_EVENT_SCENE_COLLECTION_CHANGING`
- `OBS_FRONTEND_EVENT_PROFILE_CHANGING`

Both emitted right before the change occurs. Can provide plugins with
better awareness of these critical operations happening, so that they
can react accordingly.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f