wpe: WebView and WebContext handling fixes
The WPEThreaded view is now split in 2 classes:
- WPEContextThread handles the persistent WebKit thread, where all WebKit API calls should be handled.
- WPEView: is created from the WPEContextThread. It handles the WebView and maintains the public interface on which wpesrc relies. This is the facade for the WebView, basically. It takes care of dispatching API calls into the context thread.
With these fixes it is now possible to create (and reuse) mutlple wpesrc elements during the application lifetime.
Additionally the load-failed signal handling was removed, because WebKit creates an error web-page actually when the load fails, so in the end there is no need to monitor this signal.
Fixes #1372 (closed)