UI: split left/right header buttons

- Added flexible separators to
  Clip, Graph, Dopesheet, Image, Node, Timeline, 3D View.
- Added graying out for Proportional Editing
  menus to avoid popping when right-aligned.
- Slightly re-arranged some controls,
  so they can be on correct side of the separators.

Patch by @billreynish

release/scripts/startup/bl_ui/space_node.py

@@ -51,8 +51,6 @@ class NODE_HT_header(Header):
         # Now expanded via the 'ui_type'
         # layout.prop(snode, "tree_type", text="")
 
-        NODE_MT_editor_menus.draw_collapsible(context, layout)
-
         if snode.tree_type == 'ShaderNodeTree':
             layout.prop(snode, "shader_type", text="", expand=True)
 
@@ -70,15 +68,20 @@ class NODE_HT_header(Header):
 
                 # No shader nodes for Eevee lamps
                 if snode_id and not (context.engine == 'BLENDER_EEVEE' and ob.type == 'LAMP'):
-                    layout.prop(snode_id, "use_nodes")
+                    row.prop(snode_id, "use_nodes")
+
+                NODE_MT_editor_menus.draw_collapsible(context, layout)
 
             if snode.shader_type == 'WORLD':
                 row = layout.row()
                 row.enabled = not snode.pin
                 row.template_ID(scene, "world", new="world.new")
+
                 if snode_id:
                     row.prop(snode_id, "use_nodes")
 
+                NODE_MT_editor_menus.draw_collapsible(context, layout)
+
             if snode.shader_type == 'LINESTYLE':
                 view_layer = context.view_layer
                 lineset = view_layer.freestyle_settings.linesets.active
@@ -86,6 +89,9 @@ class NODE_HT_header(Header):
                     row = layout.row()
                     row.enabled = not snode.pin
                     row.template_ID(lineset, "linestyle", new="scene.freestyle_linestyle_new")
+
+                    NODE_MT_editor_menus.draw_collapsible(context, layout)
+
                     if snode_id:
                         row.prop(snode_id, "use_nodes")
 
@@ -97,21 +103,31 @@ class NODE_HT_header(Header):
                     layout.template_ID(id_from, "texture", new="texture.new")
                 else:
                     layout.template_ID(id_from, "active_texture", new="texture.new")
+
             if snode_id:
                 layout.prop(snode_id, "use_nodes")
 
+            NODE_MT_editor_menus.draw_collapsible(context, layout)
+
         elif snode.tree_type == 'CompositorNodeTree':
+
             if snode_id:
                 layout.prop(snode_id, "use_nodes")
+
+            NODE_MT_editor_menus.draw_collapsible(context, layout)
+
+            layout.prop(snode, "use_auto_render")
             layout.prop(snode, "show_backdrop")
             if snode.show_backdrop:
                 row = layout.row(align=True)
                 row.prop(snode, "backdrop_channels", text="", expand=True)
-            layout.prop(snode, "use_auto_render")
 
         else:
             # Custom node tree is edited as independent ID block
             layout.template_ID(snode, "node_tree", new="node.new_node_tree")
+            NODE_MT_editor_menus.draw_collapsible(context, layout)
+
+        layout.separator_spacer()
 
         layout.prop(snode, "pin", text="")
         layout.operator("node.tree_path_parent", text="", icon='FILE_PARENT')