Add act and scene locators (#290)

Closes #288

Author: denismaier

schemas/styles/csl-terms.rnc

@@ -134,7 +134,8 @@ div {
   ## Locator terms that can be tested with the "locator" conditional
   ## ("sub verbo" can be tested with "sub-verbo")
   terms.locator.testable =
-    "appendix"
+    "act"
+    | "appendix"
     | "article-locator"
     | "book"
     | "canon"
@@ -149,6 +150,7 @@ div {
     | "opus"
     | "paragraph"
     | "rule"
+    | "scene"
     | "table"
     | "timestamp"
     | "title"