Fixing dispose functions, display in webview

Fixed the backend epub_document_finalize function which was caused caja to crash when you opened the document properties dialog.

Fixed the ev_window_dispose and the webview's dispose and finalise functions, now the window closes without any glib-critical errors.

Also now it is possible to read epub documents, once you have changed pages at least once, I still have to figure out where I call load_uri at document load.