Improve SceneTreeEditor usability

[RedMser]

Closes godotengine/godot-proposals#5480

A mix of usability improvements and bug fixes for the SceneTreeEditor and SceneTreeDialog. Here's an overview of the changes:

• Mark contextually relevant node using alternate text color
• Set/Clear initial selection as appropriate
• Scroll to selected node (or if none is selected, scroll to marked node)

An example of how the dialog now starts out when assigning an @export var x: Node variable:

[Mickeon]

What's the "marked" Node in this context?

[RedMser]

What's the "marked" Node in this context?

All nodes that use the theme's accent color as their text color. It's not the same as the selected node.

For example, if you have a script on node X that contains @export var foo: Node, then you try assigning a node to foo, the marked node in the dialog is X.

Similar goes for e.g. adding a new track in AnimationPlayer. The marked node there is the animation root.

[akien-mga]

Thanks!

[bruvzg]

This PR seems to be causing crashes. Probably deferred call should use ObjectID instead of raw pointer, otherwise the tree item might be removed before callback execution and pointer will become invalid.

[RedMser]

Can you show me other code locations that do something like this? Also if there's a reliable way to reproduce the crash would help.

[akien-mga]

See #90235.

[bruvzg]

Can you show me other code locations that do something like this?

Not sure if there's exactly same situations, buy you can do something like:

Add a new callback:

void Tree::_scroll_to_item_id(ObjectID p_item_id, bool p_center_on_item) {
	TreeItem *item = Object::cast_to<TreeItem>(ObjectDB::get_instance(p_item_id));
	if (item) {
		scroll_to_item(item, p_center_on_item);
	}
}

And call it like this

if (item) {
	ObjectID item_id = item->get_instance_id();
	callable_mp(tree, &Tree::_scroll_to_item_id).call_deferred(item_id, true);
}