Refactored code, created a new EvWebView type

Earlier my approach was to replace the atril view directly with a webview. However as it turned out it was extremely difficult to accomplish much tasks without the document-model. So I created an Extension of the WebKitWebView class, EvWebView so that now we can manipulate the webview as desired. Also having a separate file really helps in terms of clarity.

Please note that there are many empty functions and TODO tags right now, I shall aim to fill these up once the display part is done, probably in this week.

I also added some code on the backend to draw thumbnails when with GTK+-3.0, although I have never tested a GTK+-3.0 build.With GTK+-2.0 you won't get any build errors,  however epub backend won't work right now.

That being said, this commit is mostly because there was a lot of code that was lying in the local repository, and it was best to back it up as a failsafe.
I modified ev-window to accomodate this new data structure, and having an equivalent ev_web_view function for every ev_view function, I can achieve most functionality by filling those functions with Webkit modules.

Other than that on the backend

Other than that there were some minor changes to the code.
Although I did the above, currently I am not able to instantiate the EvWebView because of some class and object type definition errors, due to which I can't display the document.