Only save project resources that are modified

The current project-save function writes out every resource that is assigned to the project, whether it has been modified or not. Our present project users (SLAC, LANL) never modify their models once imported into a project, so this wastes alot of cycles. So this MR checks each resource's modified state and only writes those that have actually changed.

There is one exception: the code still writes all attribute resources, because we have found that some edits don't change the modified flag. The specific known case is the model association view, but there might be others.