Further attempt to make WebDAV sync more robust

This is another attempt to make the WebDAV sync more robust. In particular, this fixes an issue when the client tries to delete some files and the server responds with a 403 code. In this case, the client accepts and removes locally saved sync information so it will eventually pull the fill again from the server.

In addition, this brings back the copy menu item to get the sync log. Some debug output is removed from the sync log as well, so it should contain a much longer history of really important actions taken (which should help better understanding any sync issues).

Finally, the page menu is now automatically closed when the page stack item changes - this prevents the menu from remaining in an empty state as happened previously when using the "show sync log" option.