Trigger save on bookmark-add --toggle.

The toggle option was failing to fire the changed signal when it removed
a bookmark. This means the bookmark file would not be marked as dirty,
and would not be saved on exit/autosave (unless another change was
made).

qutebrowser/browser/urlmarks.py

@@ -280,7 +280,7 @@ class BookmarkManager(UrlMarkManager):
 
         if urlstr in self.marks:
             if toggle:
-                del self.marks[urlstr]
+                self.delete(urlstr)
                 return False
             else:
                 raise AlreadyExistsError("Bookmark already exists!")