Fix 2D viewport scrollbar contrast in modern theme

[passivestar]

Fixes the issue of poor visibility of scrollbars in the 2D viewport in the modern theme:

Before	After

Modern theme uses styleboxes for scrollbars unlike classic theme that used images with transparent pixels. Until #111975 there was no way to add space between ScrollContainer content and its scrollbar without either using images with transparent pixels or relying on border hack which only worked with opaque colors, which made them look broken in the 2D viewport. This PR uses #111975 to remove the hack and make scrollbars semitransparent.

Note that although it's now possible to add inner scrollbar margins to scroll containers, I still had to add outer margins to the 2D viewport to separate them from the viewport edges, like it's done for all viewport overlays. This has a side effect of slightly larger margins in the classic theme due to transparent pixels in them:

If necessary the scrollbars in the classic theme can also be updated later to not rely on images while keeping their look.

[YeldhamDev]

This introduces a quirk in both themes that now you can click between the scrollbar and the viewport margin, and that triggers a selection instead. It's nothing major, but something to keep in mind.