[webkit-gtk] Support clearing history

[Michael Catanzaro]

Please note, from one of my previous mails:

"WebKit does not manage browser history, so that's up to the browser 
and has nothing to do with WebKit. Clearing history works fine in 
Epiphany, for example."

Epiphany stores its history in SQLite. Other browser may do other 
things. WebKit does not have any feature for saving or managing 
persistent history, it's entirely up to the browser. Most applications 
using WebKit are actually not web browsers, and don't want any history 
at all.

Also, from one of my mails that did not make it to the list, because 
the list had been taken off CC:

"Clearing the history of the back/forward list is a security 
requirement? What is your use-case for this; what are you really trying 
to do? The back/forward list is transient session state that goes away 
when you destroy the web view. It's not persistent (unless you 
deliberately save and restore it using WebKitWebViewSessionState) and 
it's not tied in any way to the actual history functionality of web 
browsers that use WebKit."

Michael