Prevent multiple hidden Textareas

Currently a hidden ```TEXTAREA``` is created for every IText instance and remain in the DOM forever.  

This patch creates the required textarea on demand in ```enterEditing``` and destroys it in ```exitEditing```

This prevents multiple Textareas from hanging around in the DOM which can cause slowdowns in complex scenes with lots of IText instances.
Merge pull request #1111 from GordoRank/Only-add-hidden-textarea-when-in-editing-mode

Prevent multiple hidden Textareas