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.
Wow! This looks rather complicated! :) I am sure it is all working and is doingthe right thing, it's just worries me how complicated this photoshop stuff is, even with our "easy to use" engine wrapper. You probably already discussed this with @robblau but just curious - is there anything we can/should do to wrap this sort of stuff inside the engine to make this easier for clients to access? I guess it's outside the scope of this code review :)