UI: Nodes: Add Node Group Indicator and minor style tweaks

Nodes have many indicators in their header: whether they have a
preview, warnings, whether they are a node group, plus potentially
more such as if the library is linked/overriden or packed data.

Of all indicators, whether the node is a node group or not is often not
so relevant (especially when many nodes will be just node group assets)
so having an icon takes away precious real estate for other indicators.

Draw the node shape slightly different. A subtle "stack" of nodes
behind Node Groups indicate this is more than just one node.
The stack is only drawn at a certain zoom.

It is now possible to double-click a node group to enter.

It also makes some more room for the node label, since the node group
icon in the header is no longer needed.

Plus a few visual tweaks and fixes for broken/non-valid nodes.

See PR for details and screenshots.

Pull Request: https://projects.blender.org/blender/blender/pulls/145674

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -2267,6 +2267,8 @@ def km_node_editor(params):
         ("node.group_make", {"type": 'G', "value": 'PRESS', "ctrl": True}, None),
         ("node.group_ungroup", {"type": 'G', "value": 'PRESS', "ctrl": True, "alt": True}, None),
         ("node.group_separate", {"type": 'P', "value": 'PRESS'}, None),
+        ("node.group_edit", {"type": 'LEFTMOUSE', "value": 'DOUBLE_CLICK'},
+         {"properties": [("exit", False)]}),
         ("node.group_edit", {"type": 'TAB', "value": 'PRESS'},
          {"properties": [("exit", False)]}),
         ("node.group_edit", {"type": 'TAB', "value": 'PRESS', "ctrl": True},