Saving changes to multiple fields of same element about frontend_editing

[phathoang]

@jozefspisiak I started to fix it on fix-save-all-fields branch but didn't complete it, check 0cc5d0f.
The main issue is that the controller can only save 1 field at a time and I changed it. But then I went to check how Aloha did it and it is also saving 1 field at a time, or it's staging it somewhere, right after you finish editing the field. So I thought that if we will do something similar, then then my fix so the controller can save multiple fields is not needed.