Instructions to use:
-Create an Editor folder in the Assets folder.
-Drag and drop this script into the Editor folder. Now you should be able to see the MyTools menu.
-Hit play, make changes to the Position, Rotation and Scale of gameobjects in your scene.
-Click MyTools>Store Transform PlayMode Changes
-Click MyTools>Apply Transform PlayMode Changes
-Now the Position, Rotation and Scale of all your gameobjects will be the same as what they were during PlayMode.
Couple of things you should know about the tool:
1. I haven't implemented Undo functionality. So any changes you apply are permanent. You can't go back using Undo.
2. I haven't done any performance optimization at all. I was mainly focusing on the functionality itself.
3. Well this is pretty obvious...you're getting the script, so if you feel like you can modify the script and make it better in any way, go ahead. I've also added comments in the code to explain what different sections of the code do.
4. As mentioned earlier, this tool lets you store and apply only the Position, Rotation and Scale of all gameobjects in the scene. Nothing else.
Currently Unity doesn't have any feature that lets you apply changes made to transforms of all gameobjects in the scene during PlayMode. You can do it for individual components using Copy Component, but not all gamobjects in the scene as far as I know. Though, they are working on a 'Save Play Mode Changes' feature for the future, so until they do release this feature, hopefully this tool will be a little helpful to you.