GPencil: Remove Annotations panel from non-preview Sequencer areas

Fix by: Peter Fog (@tintwotin)

I did some small changes to original differential code.

Differential Revision: http://developer.blender.org/D5306

release/scripts/startup/bl_ui/space_sequencer.py

@@ -1996,6 +1996,15 @@ class SEQUENCER_PT_annotation(AnnotationDataPanel, SequencerButtonsPanel_Output,
     bl_region_type = 'UI'
     bl_category = "View"
 
+    @staticmethod
+    def has_preview(context):
+        st = context.space_data
+        return st.view_type in {'PREVIEW', 'SEQUENCER_PREVIEW'}
+
+    @classmethod
+    def poll(cls, context):
+        return cls.has_preview(context)
+
     # NOTE: this is just a wrapper around the generic GP Panel
     # But, it should only show up when there are images in the preview region