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 when an editor is open.

/cc @nathansobo and @benogle for :eyes: 
what about removing this? It should have the same effect due to the `return unless atom.themes.isInitialLoadComplete()` check in `onStylesheetsChanged`