Debug properties page doesn't support undo

Unlike the other properties pages, the Debug property page does not support multi-level undo. This means that I make multiple changes to the page I cannot undo them. This is particularly bad because you cannot discard changes due to https://github.com/dotnet/roslyn-project-system/issues/1311, meaning that if you play around with changes and you've not previously committed them to source control - you can't undo them.