Removed redundant usings, updated NuGets and fixed typos

[u1035]

What does the pull request do?

Just a small code polish with ReSharper - removed using directives that are not used, fixes typos in comments and updated Avalonia NuGet packages to latest (18->19)

Scope of this PR:

• [x] fix or update to an existing sample
• [ ] add a new sample

What is the current behavior?

Nothing changed in behavior.

Checklist

If this is a new Sample

• [ ] Added a ReadMe-file
• [ ] Updated the landing page
• [ ] Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with a link to your sample in the related documentation page

In any case

• [x] Spell-checking done
• [x] Checked if all hyperlinks work
• [x] Checked if all images are visible

Fixed issues

[u1035]

@dotnet-policy-service agree

[timunie]

LGTM 👍

Thanks for your contribution