Move MenuBar index check to an overridable method

[sdirix]

Added a protected isValidIndex method to the MenuBar class to handle index validation before setting an index.

This enables adopters to customize the validation logic, e.g., for supporting dynamic menus that may initially be empty.

Resolves #729

[sdirix]

Let me know whether I can do anything to help resolve the errors in the CI

[sdirix]

@krassowski I pushed the linter fix as a separate commit. If you prefer squashed commits, I can also happily do that

[krassowski]

Separate commits work well, we can squash on merge.

To fix Does PR have API changes? / api-changes you will need to refresh review/api/widgets.api.md by running yarn run api and committing changes.

I am not sure why Fix License Headers / header-license-fix is failing, it appears unrelated.

[krassowski]

This looks good to me, thank you @sdirix!

[sdirix]

@krassowski Please wait with the merge. While this fix enables us to build our menus, I just realized that there are some usability edge cases remaining, e.g. the menus can be opened but do not react on hover.

I had a quick glance at the code base: There are more places in which items.length are checked, which could be the culprit, so this PR is overall incomplete. If we settle on a different approach to tackle the issues, then the menubar was "enhanced" without a real benefit to anyone.

I'll come back to you. Sorry for the inconvenience!