[widgets] dropping screenshot

After discussions with Josh and Arve, I would like to propose that we 
drop the screenshot element. The rationales for the decision are:

  * screenshots bloat the package,
  * and screenshots are never used when the widget is instantiated.
  * Also, there is no way to identify on which platform the screenshot 
of the widget was taken on. This is an issue because a UA may render 
form elements used by a widget differently from one platform to another. 
Authors will want to show what a widget looks like when running on a 
platform. To show this, authors should provide distributors/galleries 
with platform specific screenshots. Having a bunch of screenshots in the 
package may confuse/mislead users in believing the widget will 
look/behave in some particular way, which may not be true on some 
platforms.

Kind regards,
Marcos