Make scrolling behave like you'd expect it to

[Friz64]

This PR adds proper scrolling logic to the scrollable widget.
Here i define a "floater" to be the element that you can drag around with your mouse to scroll.
The new behaviour is the following:

• if the user clicks off the floater, the floater jumps and attaches centered around the cursor
• if the user clicks on the floater, the floater stays where it is and attaches to the cursor position

[Friz64]

Ohhh, i completely forgot about the null renderer! Lemme fix this really quickly

[hecrj]

Thanks @Friz64! I tested it and it works nicely! 🎉

There are a couple of implementation details that I think we should address before merging.

Let me know what you think!

[hecrj]

Oh, I forgot to mention that it'd be great if you could squash the WIP commits too into one with a more descriptive message.

[hecrj]

Hey @Friz64,

I have introduced a Scrollbar type and tried to refactor things a bit. I hope I didn't break anything! Let me know what you think.

[Friz64]

Thank you for those changes, the idea of a Scrollbar type greatly improves the code quality! I just corrected a little oversight in the documentation, it now looks good to me, are we ready to merge this?

[hecrj]

Awesome! Yes, let's merge this! 🎉

Thank you again!