Fix programmatically changing index

[AlexIzh]

This PR fixes an issue with changing NavbarNotifier.index. The current code base doesn't start animation to change a visible route, it happens only when user taps on a tab. These changes run "changing tab" animation with corresponding onChanged listener for all cases (independently, if it was called programmatically or not).

#57

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing using flutter test

If you need help, consider pinging the maintainer @maheshmnj

[maheshj01]

Nevermind, I opened another PR @AlexIzh which will fix this issue. #59
I really appreciate your effort to contribute, But I really needed to get this out and published asap. I am going to close this PR.

Thank you for your contribution