Move definitions for Segments from code to TOML files

[m-d-bowerman]

          Before I review the actual code, one question: it looks like a lot of the definitions are written in code. So as we scale to additional apps, we'd have to write a new `_make_<app>_targets` function. This also makes it harder to identify where the logic is. Not saying this is a requirement to merging, this can be an improvement, but we should look into the ability to define all the Segment definitions outside the code, in a TOML file. Then the code just needs to read in and render the definitions. 

For example, in order to find out what existing user definition is for desktop, you'd have to know to look here, which is a bit buried.

Originally posted by @danielkberry in #10 (comment)