Scrollable tab bar

[bennjii]

• Closes feat: Basic Scrolling in Tab Menu #3

[emilk]

The scrolling is really nice! But:

I think top_bar_right_ui should be reverted to do what is used to do, top_bar_left_ui removed, and the ⏴ ⏵ buttons added separately (no need to use Behavior for it).

Also, the code needs comments.

[bennjii]

Updated it, theres a slight jump when scrolling to the very end as the right menu, I'm tempted to break up the main function tab_bar_ui as well, but it seems arduous.