Add vertical scrolling to bézier track editor.

[SaracenOne]

While the bézier editor likely needs a much broader UX pass, this at least adds means to scroll the track list, the absence of which makes the bézier editor basically unusable with large numbers of tracks.

Closes #74577

[fire]

Design approval. The feature is a good idea. Haven’t done a technical review.

[akien-mga]

Thanks!

[GNSS-Stylist]

@SaracenOne Sorry, but this doesn't seem to work on my setup. :( Scroll wheel of the mouse or two-finger scroll-gesture on the touchpad don't do anything unless zoom modifiers (CTRL / ALT) are pressed.

First I though this was broken again later (started working with post-RC1-version), but this new issue seemed to appear right after this commit (switched/checked out versions right before this and this).

Could this be related to OS/HW-setup? My system (From Godot's Help->Copy System Info): Godot v4.2.rc (1ba920f) - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2060 (NVIDIA; 31.0.15.4601) - Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz (12 Threads)

This is a Lenovo laptop with touchpad and an external mouse (Logitech M705). Everything else seems to work with this setup, including scrolling in other parts of the Godot's UI.

Sorry for taking so long before testing this (as the opener of the related issue), but I only just started to work again with the bézier-editor.

Don't know how this should be handled so writing this comment here...