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.
I meant to ask you that. It seems to be an established pattern in all the other loader hooks to import only in the library inside the hook instead at the top of the file. Could it be because we want these hooks to load outside the DCC?