Spellcheck in the visual editor

[azaozz]

Currently the spell check doesn't work in some browsers. As of TinyMCE 3.3 there are three options for spell checking:

• Use the browser's build-in spellchecker (all browsers except IE). A "check spelling" button is shown that attempts to toggle the browser's spell check. This toggle seems to work reliably only in Firefox for now, in the other browsers the spellcheck stays on.

• For IE there's another TinyMCE plugin that uses the (free for personal use) ​ieSpell if present or prompts the user to install it. It works pretty much like a build-in spellcheck.

• The "old-style" spellchecker that connects to a php backend for the checking and can use either Google spell or pspell/aspell.

We could use any combination of spellcheckers depending on the user's browser. If we go with the build-in spellcheckers and iespell, we can move the old-style spellchecker and its backend to a WordPress plugin for people that prefer it.

[azaozz]

(In [16405]) Revert the TinyMCE spellchecker to its old behaviour, update to tinymce_spellchecker_php_2_0_3, fixes #15268