Alternating background colours for tree items.

[AyyZerrAsa]

Alternating colours to improve item visual separation in trees. A feature in Blender and Unreal's outliners that helps readability.

For long lists of items (especially the shortcut settings), it can be difficult to read due to there being no clear boundaries between items. A subtle alternating background colour on every other item clearly shows the boundary box of items.

Before	After

Bugsquad edit: Closes godotengine/godot-proposals#9376

[passivestar]

A problem when using a custom theme with different line heights:

[Mickeon]

I feel like a grand majority of Trees do not benefit from this. There's way too much noise that does not make the additional distinction between lines worth it. Exceptions like the Shortcut tree do exist

[AThousandShips]

Did you mean to just floor the scroll? This code used to be ceil before, and for both parts

[AyyZerrAsa]

Yeah due to changes to fix the different vertical separation value issues I needed to floor here to avoid truncating to the wrong int value. Realise I also don't need the scroll logic for the empty stripes as if you can scroll, there is no empty space.

[mhilbrunner]

This is nice for tables with multiple columns (say, >2). For the others shown, simply highlighting the hovered rough is enough, IMO, and alternating lines don't serve a purpose there.

[KoBeWi]

Since the stripes make sense for some Trees and for some doesn't, maybe it would be better as regular property instead of theme one? 🤔

Other than that the implementation looks fine. It would be nice to enable stripes in some editor Trees (input map at least, not sure about others).