Reconsider/refactor computed properties

[stefandesu]

With #633, I found out that some of the computed properties in the application caused a big slowdown for everything else. As a temporary workaround, I'm using a debounced method to update those properties to replace the computed properties. This mitigates the performance issues, but it's not very elegant and it might cause delays in updating the interface. For example, filtering schemes in ConceptSchemeSelection will now take a moment until the debounced method finally gets called. I'm sure there's a better way to do this, but this will probably require some deeper changes to the application.