Support moving from/to drafts

app/src/main/java/eu/faircode/email/Core.java

@@ -591,6 +591,24 @@ class Core {
             throw new FolderNotFoundException();
         IMAPFolder itarget = (IMAPFolder) istore.getFolder(target.name);
 
+        if (EntityFolder.DRAFTS.equals(folder.type) || EntityFolder.DRAFTS.equals(target.type)) {
+            Log.i(folder.name + " move from " + folder.type + " to " + target.type);
+
+            File file = message.getRawFile(context);
+            try (OutputStream os = new BufferedOutputStream(new FileOutputStream(file))) {
+                imessage.writeTo(os);
+            }
+
+            Message icopy;
+            try (InputStream is = new BufferedInputStream(new FileInputStream(file))) {
+                icopy = new MimeMessage(isession, is);
+            }
+
+            file.delete();
+
+            icopy.setFlag(Flags.Flag.DRAFT, EntityFolder.DRAFTS.equals(target.type));
+            itarget.appendMessages(new Message[]{icopy});
+        } else
             ifolder.copyMessages(new Message[]{imessage}, itarget);
 
         // Delete source