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.
Fixed a script error when adding a feature.
Simplified geogit layer checks.
better handling of CRS code.
Adjusted the way diff lists work.
Add timestamp to notification.
Changed the way refresh works.  It's now just a button that can be hit at any time.  Auto refresh is always on.
The feature edit dialog will now respect the nillable schema property and will not let the user save null or an empty string to a field marked as non-nillable
Added billable support to the table view and fixed an issue that was preventing the table from saving edits
Added a 'null' option to the drop down inputs in the table.  Changed the UI to accommodate this and be more consistent with the rest of MapLoom
Added Nillable support to the diff panel.  Also fixed a couple rebased related issues.  There is still an issue with photos in the tableview.