Dopesheet: Add Custom Properties panel

Adds a custom property panel for the active `Action` to the Dopesheet
editor. There was previously no way to edit these properties outside of
the Python API.

This panel will show up when
`context.active_object.animation_data.action` is set.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D14646

release/scripts/startup/bl_ui/space_dopesheet.py

@@ -15,6 +15,8 @@ from bl_ui.properties_grease_pencil_common import (
     GreasePencilLayerDisplayPanel,
 )
 
+from rna_prop_ui import PropertyPanel
+
 #######################################
 # DopeSheet Filtering - Header Buttons
 
@@ -544,6 +546,20 @@ class DopesheetActionPanelBase:
         col.prop(action, "use_cyclic")
 
 
+class DOPESHEET_PT_custom_props_action(PropertyPanel, Panel):
+    bl_space_type = 'DOPESHEET_EDITOR'
+    bl_region_type = 'UI'
+    bl_context = 'data'
+    _context_path = "active_object.animation_data.action"
+    _property_type = bpy.types.Action
+
+    @classmethod
+    def poll(cls, context):
+        return context.active_object \
+            and context.active_object.animation_data \
+            and context.active_object.animation_data.action
+
+
 class DOPESHEET_PT_action(DopesheetActionPanelBase, Panel):
     bl_space_type = 'DOPESHEET_EDITOR'
     bl_category = "Item"
@@ -817,6 +833,7 @@ classes = (
     DOPESHEET_PT_gpencil_layer_adjustments,
     DOPESHEET_PT_gpencil_layer_relations,
     DOPESHEET_PT_gpencil_layer_display,
+    DOPESHEET_PT_custom_props_action,
 )
 
 if __name__ == "__main__":  # only for live edit.