Add TrayIcon sample app

[logiclrd]

What does the pull request do?

Adds a sample application demonstrating the use of <TrayIcon> in App.axaml, including an associated menu.

TrayIcon.csproj optimistically references as-yet-nonexistent version 11.2.0 of Avalonia, in the hopes that it will include the bug fix for crashing when activating tray icon menu items.

Scope of this PR:

• fix or update to an existing sample
• add a new sample

Checklist

If this is a new Sample

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

In any case

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