Functions for document info and EvPage,other changes

Summary:
 I added functions in the backend to get document info.
 Added a member to evdocument to deal with web documents(ePub) on the frontend.
 Added a webview to the window, that shall replace the Atril view.
 Due to removing the view from the main Atril window, I have given rise to various GTK-critical errors in response to signals, will be sure to suppress these in the next commit.
 Worked towards overall blending in of ePub documents.