<rdar://problem/8197701>
Add notification of when the BackForwardList changes
to aid invalidation of Back/Forward related UI elements.

Reviewed by Anders Carlsson.

WebKit2: 

* UIProcess/API/C/WKPage.h:
Add didChangeBackForwardList to the WKPageLoaderClient. This 
fires whenever an item is added or removed from the back forward
list or when the cursor changes position.

* UIProcess/WebBackForwardList.cpp:
(WebKit::WebBackForwardList::addItem):
(WebKit::WebBackForwardList::goToItem):
* UIProcess/WebLoaderClient.cpp:
(WebKit::WebLoaderClient::didChangeBackForwardList):
* UIProcess/WebLoaderClient.h:
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::didChangeBackForwardList):
* UIProcess/WebPageProxy.h:
Pipe changes to the WebBackForwardList up to the page load client.

WebKitTools: 

* MiniBrowser/mac/BrowserWindowController.m:
(_didStartProvisionalLoadForFrame):
(_didCommitLoadForFrame):
(_didFailLoadWithErrorForFrame):
(_didChangeBackForwardList):
(-[BrowserWindowController awakeFromNib]):
* WebKitTestRunner/WebKitTestRunner.xcodeproj/project.pbxproj:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@63590 268f45cc-cd09-0410-ab3c-d52691b4dbfc