ZetboxDataGrid: improve logging
TestApp: add tester, improve list
NavigationScreen: fix wrong cast
GridSupport: reset AllowAddNew to workaround DataGrid bug
InstanceList: inline editable lists and grids should not open items on double-click
ErrorListViewModel: fix display of errors which do nat have a direct data object parent
WPF Grid: Style the NewItemPlaceholder properly
WPF Grid: make columns readonly as required