Enabled deletion of photos in attribute edit dialog.
Improvements to server and layer handling.

- Improved style for add layer dialog to avoid double scroll bars.
- Added a button to clear the filter.
- Filter gets cleared when dialog is closed.
- Added "always anonymous" option to credentials dialog to prevent asking every time the map is loaded.
Prompt for user credentials again after editing the url of a server.
Provide an error dialog when a script error occurs.
Avoid script error when adding remote servers.