Warning cleanup

[MrJul]

What does the pull request do?

This PR fixes several dozens of compiler and external analyzer warnings in the desktop solution.

How was the solution implemented?

Apart from the obvious fixes for the most common issues (nullability mismatch, wrong documentation cref, unused event), the notable changes are:

• Disabled trim warnings for netstandard2.0. The suppressions don't work correctly for it, and trimming is done on a real TFM anyways. Trim warnings are still enabled as errors for net6.0, so nothing should be missed.
• Added an AsAsyncEnumerable method to transform IEnumerable to IAsyncEnumerable without warnings.

Remaining warnings

• Warnings in XAML loader, fixed in Warning fixes kekekeks/XamlX#95 (pending merge).
• Warnings coming from Avalonia's analyzers itself, will be fixed in another PR for easier review.
• Warnings in samples, unit tests and mobile projects, will be fixed in another PR.