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 theme load to complete before measuring so it is only called once at startup and removes **200-300 millseconds** from startup time when an editor is open.

/cc @nathansobo and @benogle for :eyes: 
`stylesheet?` for consistency?