Add "inner item margin" to Tree-related controls

[joao-pedro-braz]

Describe the project you are working on

Godot Editor

Describe the problem or limitation you are having in your project

Normal, focus and hover panels on Tree-related control items with icons currently don't have any horizontal separation from their borders. As a result, icons appear too close to the item's edge:

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Implemented in godotengine/godot#75460, an "inner_item_margin" property to the base Tree control allows the horizontal spacing to be specified, which in turn can be used by the editor to solve the previously mentioned issue:

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Again, implemented in godotengine/godot#75460, but in technical terms it's basically updating the effective Tree Item's rect (size and position):

rect.set_size(rect.get_size() - Size2i(theme_cache.inner_item_margin, 0));
rect.set_position(rect.get_position() + Size2i(theme_cache.inner_item_margin, 0));

If this enhancement will not be used often, can it be worked around with a few lines of script?

Not as far as I'm aware

Is there a reason why this should be core and not an add-on in the asset library?

Involve changes to the rendering code of a Control.