Add theme type variations for secondary Trees and ItemLists

[passivestar]

This aims to address the issue raised in passivestar/godot-minimal-theme#8

This PR has no effect on the default theme.

One of the reasons I made a theme was to simplify the layout by removing some of the docks' background panels. But there's an issue with that.

In Godot some trees occupy the majority of their containers and show what you could call "primary content", like the trees in the Scene and the FileSystem docks, or the main area of the File Dialog. Meanwhile some other trees (and itemlists) show secondary "sidebar" content, like the ones on the left in the Shader Editor, Profiler, Create Dialog, etc. Those sidebar trees and lists need to have background panels to be visually separated from the rest of the content, but it's impossible to have different background panels for them that wouldn't affect "primary" lists

This PR adds type variations for secondary lists to make it possible to theme them separately

Old	New

Theme for testing: godot-minimal-theme.zip

[passivestar]

Moved them to the editor theme as per suggestion

[Geometror]

This is great! Code looks fine, I'm only a bit sceptical about the "Side~" prefix. What do you think about "Sub~" (or "Secondary~"/"Sec~")?
I'd also consider using a suffix instead to make it a tiny bit clearer that it's a theme type variation and not another type, so e.g. "TreeSecondary" and "ItemListSecondary". IMO having a longer name for a theme type variation isn't a problem.

[passivestar]

This is great! Code looks fine, I'm only a bit sceptical about the "Side~" prefix. What do you think about "Sub~" (or "Secondary~"/"Sec~")?

Decided to go with Secondary, 'Sub'Tree could imply that it belongs to a tree hierarchy of sorts. Secondary is ok though

Applied the suggestions and tested again with an updated theme, works as expected

Upd: Also added a previously missed itemlist of editor help overview