Adding to schema does not remove error

[SteveMarvin]

We now get a validation error indicating that a mark is not in the schema, and we get the button to add it, but adding it does not remove the validation error or the error count in the resource window

you have to save, close the file and reopen it for the error to be gone

[NickLaiacona]

Ahh, forgot this was already here - but I found that it does add it once you click away, see #602 .