feat: Multi-level titles

[matthew-carroll]

Most slide show presentations that I put together have multiple levels of titles.

For example, one of my current presentations has the following titles at various levels:

• The Art of Flutter Testing
  • Dispelling Common Myths
    • Tests check the correctness of your code
    • You should strive for 100% test coverage
  • Avoid Team Confusion
    • Avoid Ambiguous Terms
    • Speak in Terms of Tooling
    • Don't use 3rd Party Test Runners
  • Tour of Test Tools
    • Dart Language tests
      etc, etc, etc

As you can see, I have 3 distinct levels of titles, each of which should have different styling.

I think it would make sense for the default flutter_deck slides to include support for different levels of titles. I'm sure it's possible to build these myself, but this requirement is so ubiquitous that if flutter_deck is going to provide a TitleSlide, it should let you specify what level of title. The top level would be the presentation title slide, the first level is a section, and the second level is a subsection.

Or, if passing an arbitrary level to TitleSlide isn't ideal, there could be two slides added: SectionTitleSlide and SubsectionTitleSlide.