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.
Ahh, good question! Previously, we would stash multiple engines into a single hook python file. This meant that the same hook is used in multiple engines.  In that case, the imports need to be local. But with a lot of the new hooks, the hook file itself is scoped (by its file name) so it is guaranteed not to run in other engines! So imports can be up the top!