Add start/introduction screen and basic pre-made chain templates

[Kim2091]

This would be temporary, just to make things easier for new users. This would likely involve adding a "start" screen similar to most photo editors, which would allow you to easily pick these templates.

Ideally it'd also have a preview of what the chains look like, likely an image taken of the chain that's saved with the chain file. This could be used beyond just the beginner templates of course.

[RunDevelopment]

I think this might be easy to solve once we have tabs. We could simply have a "Welcome" tab that shows things like templates, recently opened/saved chains, and so on.