Provide better default export behavior for simple non-artboard docs

[mcilroyc]

When opening a JPG file (for example) in DS, it requires several steps to do a simple export. This should be better.

This is a spin off of #3710

from seth: a few ideas… 1) unlock bg layers for JPGs 2) Auto create an export for non-PSD image formats 3) Then, I would think, if we could show the asset, possibly in the artboard column

• Scale= 1
• Suffix= none
• Settings= file format ? same as file format : png