Improve the WYSIWYG/Source tab switch

Improve the WYSIWYG/Source tab switch to:

• be able to switch to source tab before the rich text area finishes loading
• be able to cancel a switch (i.e. come back to the previous tab is the current tab take too long to load)
• handle the case when the conversion from HTML to source fails
• handle the case when the conversion request (in both ways) fails (e.g. server not found)