[MRG] New saving UI

Changes in this PR:

* WebAnnotator got extension preferences in a standard location;
* "Save and Export" dialog is replaced by 2 menu items: "Save as.." and "Export as..";
* filenames for saved files are auto-suggested based on their URL and title;
* auto-export feature is added (it allows doing the export in addition to save on Save as command);
* folders with images/css/other resources are named automatically;
* there is an option to make all links absolute (so that page will display as before after saving without creating extra local files);
* there is an option to add `<base>` html tag to page as an alternative for making links absolute (sometimes it is important to preserve relative links);
* some functions are renamed for clarity;
* a bug in DTD import is fixed.