Refactor annotation flags code

This patch makes it possible to set and get all possible flags that the PDF specification defines. Even though we do not support all possible annotation types and not all possible annotation flags yet, this general framework makes it easy to access all flags for each annotation such that annotation type implementations can use this information.

We add constants for all possible annotation flags such that we do not need to hardcode the flags in the code anymore. The `isViewable()` and `isPrintable()` methods are now easier to read. Additionally, unit tests have been added to ensure correct behavior.

This is another part of #5218.

src/shared/util.js

@@ -48,6 +48,19 @@ var AnnotationType = {
   LINK: 3
 };
 
+var AnnotationFlag = {
+  INVISIBLE: 0x01,
+  HIDDEN: 0x02,
+  PRINT: 0x04,
+  NOZOOM: 0x08,
+  NOROTATE: 0x10,
+  NOVIEW: 0x20,
+  READONLY: 0x40,
+  LOCKED: 0x80,
+  TOGGLENOVIEW: 0x100,
+  LOCKEDCONTENTS: 0x200
+};
+
 var AnnotationBorderStyleType = {
   SOLID: 1,
   DASHED: 2,