Indirect post title in editor

Closes #3179
- Add `titleScratch` property to `PostModel`.
- Changed references to `title` to `titleScratch`
@jaswilli Would you OK this change?  Since `titleScratch` isn't an attribute, we can't detect a change on it like we could with title. However, the only reason we're inside of `titleObserver` is that the title changed, right? So, redundant?