[GTK] Add WebKitWebView::context-menu-dismissed signal to WebKit2 GTK+ API

[GTK] Add WebKitWebView::context-menu-dismissed signal to WebKit2 GTK+ API
https://bugs.webkit.org/show_bug.cgi?id=90386

Reviewed by Martin Robinson.

* UIProcess/API/gtk/WebKitWebView.cpp:
(webkit_web_view_class_init): Add context-menu-dismissed signal.
(contextMenuDismissed): Emit context-menu-dismissed signal.
(webkitWebViewPopulateContextMenu): Connect to deactivate signal
of the GtkMenu and emit WebKitWebView::context-menu-dismissed when
the gtk menu is deactivated.
* UIProcess/API/gtk/WebKitWebView.h:
(_WebKitWebViewClass): Add virtual methos for
context-menu-dismissed signal.
* UIProcess/API/gtk/tests/TestContextMenu.cpp:
(testContextMenuDismissed):
(beforeAll):
* UIProcess/gtk/WebContextMenuProxyGtk.h:
(WebKit::WebContextMenuProxyGtk::gtkMenu): Get the GtkMenu built
for the context menu proxy.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121771 268f45cc-cd09-0410-ab3c-d52691b4dbfc