Loader now adds a layer instead of opening a file in Photoshop

This branch implements the Add To Layer action for the loader. It supports both images and PSDs.
The 3 main tasks were:
- Renaming the Open File action to Add To Layer.
- Adding a check to make sure there is a current document.
- Using the Place action to create the layer.
Do we really want to get rid of the open file action? that means we are removing a feature - a feature that our clients may be using today! I read the ticket and I think it says that the *default action* should be this new layer method, but the open file should still be available. Maybe double check with @ryanmayeda !