added features to be able to provided the selected image on tab item

[ghost]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

feature

⤵️ What is the current behavior?

you can set a tab bar item's image, but not the selected image

🆕 What is the new behavior (if this is a feature change)?

you can provide the selected image icon

💥 Does this PR introduce a breaking change?

I don't think so because the string for selected image can be null and if it's null then it's not set on the tab bar item

🐛 Recommendations for testing

I updated the code in the play ground for testing

📝 Links to relevant issues/docs

I mentioned this in slack to the team

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated (docs style guide)
• Nuspec files were updated (when applicable)
• Rebased onto current develop

[mention-bot]

@biozal, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nmilcoff, @Stephanvs and @tomgilder to be potential reviewers.

[nmilcoff]

Only thing to notice: It is a breaking change. All apps that have overriden MvxTabBarViewController.ShowTabView and MvxTabBarViewController.SetTitleAndTabBarItem will break

[ghost]

@nmilcoff would the recommendation be to leave current methods in place and just have them call new methods with the new signature? Then this wouldn't break as the old method signatures would still be there and they would simply call the new ones.