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