Fix: Grease Pencil data panels not working with pinning

Note: Things were working fine if you were to pin the Grease Pencil
object, but not if you were pinning the GP data.

In too many poll functions context.object was being requested when
a simple context.gpencil_data would suffit.

Panels that are still not showing in pinning:
* DATA_PT_gpencil_display.

The panel needs to be split in sub-panels, leaving all object-dependent
properties in its own panel so we can poll it away, while showing the
rest.

* * *

This commit doesn't handle greasepencil material. In this case I
recommend we do as properties_material.py and have a generous poll(),
followed by different drawing logics whether or not we have an object.

release/scripts/startup/bl_ui/properties_data_gpencil.py

@@ -34,6 +34,16 @@ class DataButtonsPanel:
     bl_region_type = 'WINDOW'
     bl_context = "data"
 
+    @classmethod
+    def poll(cls, context):
+        return context.gpencil_data
+
+
+class ObjectButtonsPanel:
+    bl_space_type = 'PROPERTIES'
+    bl_region_type = 'WINDOW'
+    bl_context = "data"
+
     @classmethod
     def poll(cls, context):
         return context.object and context.object.type == 'GPENCIL'
@@ -46,8 +56,7 @@ class LayerDataButtonsPanel:
 
     @classmethod
     def poll(cls, context):
-        return (context.object and
-                context.object.type == 'GPENCIL' and
+        return (context.gpencil_data and
                 context.active_gpencil_layer)
 
 
@@ -103,14 +112,7 @@ class DATA_PT_gpencil_datapanel(Panel):
 
     @classmethod
     def poll(cls, context):
-        if context.gpencil_data is None:
-            return False
-
-        ob = context.object
-        if ob is not None and ob.type == 'GPENCIL':
-            return True
-
-        return False
+        return context.gpencil_data
 
     @staticmethod
     def draw(self, context):
@@ -294,7 +296,7 @@ class GPENCIL_UL_vgroups(UIList):
             layout.label(text="", icon_value=icon)
 
 
-class DATA_PT_gpencil_vertexpanel(DataButtonsPanel, Panel):
+class DATA_PT_gpencil_vertexpanel(ObjectButtonsPanel, Panel):
     bl_space_type = 'PROPERTIES'
     bl_region_type = 'WINDOW'
     bl_context = "data"
@@ -359,7 +361,7 @@ class DATA_PT_gpencil_strokes(DataButtonsPanel, Panel):
         layout.prop(gpd, "use_adaptive_uv", text="Adaptive UVs")
 
 
-class DATA_PT_gpencil_display(DataButtonsPanel, Panel):
+class DATA_PT_gpencil_display(ObjectButtonsPanel, Panel):
     bl_label = "Viewport Display"
     bl_options = {'DEFAULT_CLOSED'}