Generalize layers as merge nodes to enable adjustment layers (#1712)

* WIP, backward traversal issues

* Fix some tool issues

* Remove debugging

* Change some indices

* WIP: new artboard node

* WIP: add artboard node

* WIP: Artboard node and create_artboard

* WIP: Artboard node implementation complete

* WIP: Artboards input for output node

* Complete Artboard node

* Generalize LayerNodeIdentifier,  monitor_nodes support for Artboard node, adjust ResizeArtboard/ClearArtboards, move alias validation to Rust

* Fix misaligned artboard click targets

* Generalize/clarify create_layer and insert_between

* non-negative dimensions for resize_artboard

* Show artboards in layer panel

* Generalize create_layer for layer output node

* Generalize delete_layer/delete_artboard to NodeGraphMessage::DeleteNodes. Fixed upstream flow Iter

* remove old primary_input function

* Vertical node visuals, remove is_layer function, rename Layer node to Merge node, toggle display as layer

exposed_value_count type fix

Vertical node visuals, remove is_layer function, rename Layer node to Merge node, toggle display as layer

* Fix demo artwork

* Layer display context menu

* Automatically select artboard, fix warnings

* Improvements to context menu and layer invariant enforcement

* Remove display_as_layer and update load_structure

* Improve load_structure to show more layers, improve FlowIter, improve layer naming, layer rearrangement validation.

* Clean up demo artwork using generalized layers

* Improve design of Layers panel and graph nodes

* MoveSelectedLayersTo rewrite to support generalized layer nodes

* Include artboards in deepest_common_ancestor, fix resize_artboard/delete_artboard, sync artboard tool to layer panel

* MoveSelectedLayersTo adjustments

* Sync non layer node visibility with metadata

* Include non layer nodes when moving/creating layer

* Fix group layers and get_post_node_with_index

* Include non layer nodes in UngroupSelectedLayers

* GroupSelected for all selected nodes, UnGroupSelected position adjustments

* Add grouping for layers in different folders

* Fix hidden layers

* Prevent node from connecting to itself, fix undo automatic node insertion,

* Fix undo CreateEmptyFolder, fix grouping nested layer nodes

* Formatting

* Remove test and check if node is layer from network

* Fix undo group layers

* Check off roadmap

* MoveUpstreamSiblingsToChild adjustments

* Replace tabs with spaces, remove mut from argument

* Final code review pass

---------

Co-authored-by: 0hypercube <0hypercube@gmail.com>
Co-authored-by: Keavon Chambers <keavon@keavon.com>

website/content/features.md

@@ -113,7 +113,7 @@ Always on the bleeding edge and built to last— Graphite is written on a robust
 		<div class="informational ongoing heading" title="Began February 2024" data-year="2024">
 			<h3>— Alpha 3 —</h3>
 		</div>
-		<div class="informational ongoing" title="Development Ongoing">
+		<div class="informational complete" title="Development Complete">
 			<img class="atlas" style="--atlas-index: 3" src="https://static.graphite.rs/icons/icon-atlas-roadmap__2.png" alt="" />
 			<span>Stackable adjustment layers</span>
 		</div>