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_evaluation_selectability_a.py

@@ -38,7 +38,7 @@ class UnitTesting(ViewLayerTesting):
 
         layer_collection_mom.enabled = False
         bpy.context.scene.update()  # update depsgraph
-        cube.select_set('SELECT')
+        cube.select_set(True)
 
         self.assertTrue(cube.visible_get(), "Cube should be visible")
         self.assertTrue(cube.select_get(), "Cube should be selected")