NLA Editor: Operator to add AnimData to selected objects so that they can appear

This commit introduces an operator in the Add menu - this operator ensures that
all selected objects have AnimData attached to them (even if they don't have any
actions/drivers yet). By doing this, these objects can at least appear in the
NLA Editor, which will allow them to have strips added to them in future without
having to create throwaway actions first (NOTE: there's still some stuff coming
to allow that).

Also, renamed NLA_OT_delete_tracks -> NLA_OT_tracks_delete

release/scripts/startup/bl_ui/space_nla.py

@@ -167,6 +167,9 @@ class NLA_MT_add(Menu):
         layout.separator()
         layout.operator("nla.tracks_add").above_selected = False
         layout.operator("nla.tracks_add", text="Add Tracks Above Selected").above_selected = True
+		
+        layout.separator()
+        layout.operator("nla.selected_objects_add")
 
 
 class NLA_MT_edit_transform(Menu):