Make inspector spacing more themable

[passivestar]

Some tweaks to make it easier to establish visual hierarchy in the inspector:

Before	After

• Takes the content margins of EditorInspectorCategory/styles/bg into account when calculating EditorInspectorCategory min height to allow independent padding inside of it. It was only controllable through Tree v_separation, making it impossible to set independently from all of the trees in the editor
• Exposes EditorInspector main vbox separation to theming to allow space between inspector categories and sections
• Makes horizontal spacing of EditorInspectorSection themable (space between the arrow and the text)
• Ensures that text is vertically centered in categories and sections (with floor() they were 1px shy, you can notice if you look real close at those category sections on the left picture)

Note: when using vertical separation there's double separation below some node categories (like Node and Node3D) because there's a 0-height vbox hidden in there (not yet sure what it's for, probably some warning). I think VBoxContainer shouldn't do separation between 0-size controls, this is perhaps something to fix in a future PR

I also want to make section backgrounds themable to support rounded corners but they are drawn immediately through draw_rect because they're using different alphas depending on the nesting level. Maybe I should generate 5-6 stylebox variations for different levels. Something for a future PR as well

[KoBeWi]

This makes the section headers bigger in the default theme. Is this intentional?
It does not look bad, but it takes more space.

I think VBoxContainer shouldn't do separation between 0-size controls

Nah, it's fine like that. If you don't want separation, the Control should be hidden.

[passivestar]

This makes the section headers bigger in the default theme. Is this intentional?

Not intentional, will fix

[AThousandShips]

I'd call it "spacing" and not "whitespace" as this doesn't affect label or other text whitespace, so "spacing" or "separation" would be a less confusing naming IMO

[passivestar]

meant whitespace as it'd be used in design to refer to empty space, but I understand how it can be confusing

[akien-mga]

Thanks!