Ensure ApplicationShellOptions are properly used for shell init

[martin-fleck-at]

What it does

Ensure ApplicationShellOptions are properly used for shell init
Fixes #12982

How to test

1. Use a custom re-binding in your application, e.g., rebind(ApplicationShellOptions).toConstantValue(<ApplicationShell.Options>{ leftPanel: { initialSizeRatio: 0.5 } }); or replace the binding in Theia with something that has values (https://github.com/eclipse-theia/theia/blob/master/packages/core/src/browser/frontend-application-module.ts#L163)
2. Ensure your workspace layout is reset to default.
3. Expand the left sidebar and see that the right ratio is used.

Follow-ups

None.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tortmayr]

IMO we could resolve the options directly in the constructor. There is no need to this in postConstruct because we don't rely on any injected services. This would also remove the need to expose the partial options as protected property.

[martin-fleck-at]

Thank you for the having a look! Good point, I updated the PR with this simplification.

[tortmayr]

Thanks for the quick fix. Change works great and custom shell options are now properly picked up and used for shell init. 👍🏼