Simplify the DFW implementation

[lessbloat]

I was sad to see that distraction free writing ​didn't get picked up for 3.6. I decided to play around with a rough proof of concept to see how difficult it would be to use the existing TinyMCE instance for DFW.

Benefits of handling it this way include:

• It eliminates the need (and complexity) of juggling multiple instances of TinyMCE
• We'll be able to remove a bunch of the old DFW code
• It makes the transition between regular mode and DFW mode much more smooth
• It can be as fully featured as we want it to be

[lessbloat]

23630.diff​ is a rough first pass at this. The code still needs to be cleaned up, and I'm sure there are bugs. To activate it, click the new DFW icon under the "visual & text" tabs.

Here's a ​quick screencast of it in action.

[lessbloat]

23630.1.diff​ is a refresh of 23630.diff​ with a few additional minor tweaks.

[azaozz]

This is starting to look really good, hoping we can polish it a bit more and add it in the next release.

[iseulde]

We've changed the DFW mode in 4.1. :)