Refactor background.dart in Dart Debug Extension

[elliette]

This shouldn't affect or change any functionality. This is simply a refactor of the background.dart file to make it easier to read and understand.

The changes are:

1. Switch anonymous function callbacks to named functions so it's easier to understand what they are doing
2. Re-use some of the named functions in multiple places to reduce code duplication
3. Add lots of comments

[annagrin]

Thanks Elliott, great readability changes! Add some comments to possibly add more.

[annagrin]

Thanks you Elliott, this is much easier to read! LGTM!