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_scene_copy_e.py

@@ -26,7 +26,7 @@ class UnitTesting(ViewLayerTesting):
         layer = bpy.context.view_layer
 
         original_cube = layer.objects.get('Cube')
-        original_cube.select_set('SELECT')
+        original_cube.select_set(True)
         self.assertTrue(original_cube.select_get())
 
         bpy.ops.scene.new(type='FULL_COPY')