Support file save triggered from the Firefox integrated version.

Related to https://bugzilla.mozilla.org/show_bug.cgi?id=1659753

This allows Firefox trigger a "save" event from ctrl/cmd+s or the "Save
Page As" context menu, which in turn lets pdf.js generate a new PDF if
there is form data to save.

I also now use `sourceEventType` on downloads so Firefox can determine if
it should launch the "open with" dialog or "save as" dialog.

web/firefoxcom.js

@@ -115,7 +115,7 @@ class DownloadManager {
     );
   }
 
-  download(blob, url, filename) {
+  download(blob, url, filename, sourceEventType = "download") {
     const blobUrl = URL.createObjectURL(blob);
     const onResponse = err => {
       if (err && this.onerror) {
@@ -130,6 +130,7 @@ class DownloadManager {
         blobUrl,
         originalUrl: url,
         filename,
+        sourceEventType,
       },
       onResponse
     );
@@ -231,6 +232,17 @@ class MozL10n {
   }
 })();
 
+(function listenSaveEvent() {
+  const handleEvent = function ({ type, detail }) {
+    if (!PDFViewerApplication.initialized) {
+      return;
+    }
+    PDFViewerApplication.eventBus.dispatch(type, { source: window });
+  };
+
+  window.addEventListener("save", handleEvent);
+})();
+
 class FirefoxComDataRangeTransport extends PDFDataRangeTransport {
   requestDataRange(begin, end) {
     FirefoxCom.request("requestDataRange", { begin, end });