Rewrite FileDigester to tidy up code, be able to delete files in OPFS onPageHide and on abort of file transfer

public/scripts/util.js

@@ -619,4 +619,27 @@ function isUrlValid(url) {
     catch (e) {
         return false;
     }
+}
+
+// polyfill for crypto.randomUUID()
+// Credits: @Briguy37 - https://stackoverflow.com/a/8809472/14678591
+function generateUUID() {
+    return crypto && crypto.randomUUID()
+        ? crypto.randomUUID()
+        : () => {
+            let
+                d = new Date().getTime(),
+                d2 = ((typeof performance !== 'undefined') && performance.now && (performance.now() * 1000)) || 0;
+            return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, c => {
+                let r = Math.random() * 16;
+                if (d > 0) {
+                    r = (d + r) % 16 | 0;
+                    d = Math.floor(d / 16);
+                } else {
+                    r = (d2 + r) % 16 | 0;
+                    d2 = Math.floor(d2 / 16);
+                }
+                return (c == 'x' ? r : (r & 0x7 | 0x8)).toString(16);
+            });
+        };
 }