refactor: cleanup and document Scrollbar

[Nerixyz]

Working on #5329, I realized that Scrollbar needed some documentation and a rename of largeChange. Then I saw some unused functions and clang-tidy warnings, so I decided to refactor the whole class.

The commits and documentation should work on their own - if you're missing information, please point that out and I'll add more docs.

Since I added some documentation, I decided to add a doxygen target (if available), so I don't have to re-add that every time I want to generate documentation. It's not meant to be pretty - we could use breathe for that. But it's not really a priority since Chatterino isn't a library.

[Pizzabelly]

Nice. I could suggest renaming resetMaximum() to resetBounds(). Maybe also a note about getRelativeCurrentValue() being the current position adjusted to the bounds.

[pajlada]

I added some tests for addHighlightsAtStart that run on current master, but fails with this PR

[dnsge]

I have a couple more things that would be nice to refactor, see Nerixyz#22 :)