[3.x] Add support for partial custom editor themes

[YuriSizov]

This allows custom editor themes to be incomplete by merging them with the generated theme. This ensures that every theme item expected by the editor exists in some form, and allows custom themes to be more granular. For example:

This result is achieved with a theme resource that looks something like this:

[gd_resource type="Theme" load_steps=2 format=2]

[sub_resource type="StyleBoxFlat" id=1]
content_margin_left = 8.0
content_margin_right = 8.0
content_margin_top = 5.0
content_margin_bottom = 18.0
bg_color = Color( 0.172549, 0.341176, 0.137255, 1 )
border_width_left = 2
border_width_top = 2
border_width_right = 2
border_width_bottom = 12
border_color = Color( 0.686275, 0.580392, 0.215686, 1 )

[resource]
Button/styles/normal = SubResource( 1 )

This should address problems like the one shown in #51414. This might also be useful for merging the default theme and the custom project theme for the projects made with Godot so there is less time spent on lookups, but that's a story for another time.

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This also removes unnecessary theme operations in the canvas item editor plugin. It was the only place in the engine that relied on copying the theme, which is completely unnecessary. From my superficial testing the operation took 630-640 microseconds and now takes 20. This only happens once, as the editor is created, but it may remove some hitching for some users, I think. Either way, the theming code is better now, plus the styling applied can now be customized with the editor theme.

[akien-mga]

Thanks!