RNA: Object.select_set use boolean, only select

- Was setting active state, making it necessary to backup/restore
  active object in cases where this isn't needed.
  Existing scripts are explicitly setting the active object when needed.

- Use a boolean select arg (toggle selection wasn't used anywhere).

- Add an optional view layer argument since scripts should be able to
  operate outside the user context.

tests/python/view_layer/test_make_single_user.py

@@ -36,7 +36,7 @@ class UnitTesting(ViewLayerTesting):
                 master_collection.collections[0])
 
         view_layer.collections.link(master_collection)
-        ob.select_set('SELECT')
+        ob.select_set(True)
 
         # update depsgraph
         scene.update()