New Widgets API doesn't allow to return feedback on save

[Denis-de-Bernardy]

When js is enabled, and you save a widget, there are several issues with the current implementation:

• If an error occurs (in the script or wherever), it's difficult to know
• You cannot return an error message (short of using js form validation) to a user when he enters erroneous values
• If the user enters values that get sanitized or auto-corrected by the update function, the widget still shows the junk entered by the user upon saving

Suggested enhancement: output the results of update() and form() on the ajax, and replace the widget's current contents with their updated value.

Marking this as a defect, rather than an enhancement, due to how complicated it makes writing a widget form without using Firebug.

In [11498]