Hidden TinyMCE controls in any but the first tab needlessly trigger unloadProtection

If a hidden textarea or possibly a hidden text field is defined as a tinyMCE control in any but the first tab on a tabbed form unloadProtection warnings appear when they need not to.

## Steps to reproduce

* Define a tabbed form with a hidden tinyMCE textarea in the second tab.
* Create a new item with this form
* Open the item edit form
* Select the second tab (do not change anything)
* Try to navigate away from the from

## Behavior

After completing the steps above, the unload protection message will pop up, even though nothing has changed

## Expected behavior

The unload protection message will not pop up, as nothing has changed

## Cause

I'm not quite sure yet why this happens. It happens to me regularly on my forms, but the steps described above do not always trigger the error. I tried doing it hastily, slowly, with Chrome, Firefox. The error happens about every second try.

## Fix

My change uses the getContent function of tinyMCE, which always yields the actual value. Without this change, the element.value may contain tinyMCE code like this:

```html
<p><br data-mce-bogus="1"></p>
```

With my fix the error does not appear anymore at all.

## Notes

I'm not totally sure this should be solved like this, it's just the most obvious place / fix I could find. I'm also not sure if I need to do anything else to get this fix into Plone. Please let me know what you think and if there's something I'm missing / not doing right.