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.
Just curious - why are there brackets around the `("Please open a document first.")`?