Multiple bug fixes

Hi there, I've just fixed a bunch of bugs, haven't added any new functionality. There's a couple of cases where I changed around the code formatting too.

* Now reference `$this` rather than `__CLASS__` - was causing PHP notices in some cases
* Splitdown scripts now only load on editing post pages, not all pages (was breaking Appearance > Menus)
* Handles blank `$_POST` values on save
* Has margin below editing wrapper so following widgets aren't touching
* Fixed spelling error for `destination` in JS
* Updates markdown at start to prevent blank posts being saved (the content that's previewed is the one that's saved)