When the height of the editor couldn't fit the sidebar tabs, they would shrink
to a size too small to read the tab label.
This change matches the behaviour with the Properties Editor navigation bar,
by introducing the following improvements:
* Avoid truncating tab labels.
* Allow scrolling when tabs don't fit.
Behaviour is similar to how scrolling works in the Properties Editor navigation
bar, supporting mouse wheel up/down and MMB, and switching tabs with
`Ctrl+Wheel Up/Down`.
Pull Request: https://projects.blender.org/blender/blender/pulls/105355
c0e757a713