autotools: Adapt autotools for using WebKit2

https://bugzilla.gnome.org/show_bug.cgi?id=686376
yelp: Change headers files
yelp-view: Replace "navigation-policy-decision-requested" signal by "decide-policy" signal

WK2 only has a signal for policy decisions.
yelp-view: Replace "script-alert" signal by "script-dialog" signal
yelp-view: Replace "resource-request-starting" signal by "resource-load-started" signal

I have to use new class WebKitURIRequest methods.
yelp-view: Replace "document-load-finished" signal by "load-changed" signal

"load-changed" is a generic signal that receives a type parameter. I have add if
condition to return any execution of the handler that doesn't correspond to a
LOAD_FINISHED event.
In addition we have a problem (again) with DOM modification that should be FIXED
someway. Maybe WebExtensions is a good choice this time.
yelp-view: Rename WebKitWebSettings to WebKitSettings
yelp-view: Implement view_print_action using WebKit2 API
yelp-view: Use WKFindController instead of deprecated search functions in WKWebView
yelp-view: Adapt properties of WebSettings to WK2 Settings

Remove enable-universal-access-from-file-uris that doesn't
exist any more and rename default-encoding to default-charset.
yelp-document: Avoid GLib warning on referencing null object
yelp-view: Replace "populate-popup" signal by "context-menu" signal

New WK2 API HitTestResutl class doesnt have a reference to the DOM
node so we cant know using this property if we are inside a code block
or the name of a link.
yelp-view: Implement web extension to deal with DOM tree
yelp-view: Fix view_show_error_page

Substitute webkit_web_view_load_string call for webkit_web_view_load_html
and remove navigation requested signal unblock.
yelp-view: Implement custom uri schemes

We should deal with the uris that Yelp undestand and the uris that Webkit undestand so we have created functions to convert from one to the others. In addition, we should add a hack to be able to load absolute uris. When we have a help:gnome-help/... uri on our document and the current page has the same scheme (help) WebKit interprets this uri as relative and it builds a different uri. To fix this instead of use a help scheme we use a bogus-help schme so WebKit interprets the uri as absolute.
yelp-view: Implement web extension to load resources