Wait for themes to load before measuring

While profiling I noticed that `EditorComponent::onStylesheetsChanged` was called around 25 times as each theme stylesheet was loaded, taking a total 200-300 milliseconds when an editor was open at startup.

Now the component waits for the initial load to complete before measuring so it is only called once at startup and removes the 200-300 millseconds from startup.

/cc @nathansobo and @benogle for :eyes: