Add missing Screen.Equals/GetHashCode overrides

[MrJul]

This PR adds missing overrides for Equals(object) and GetHashCode() on Screen.
This has two purposes: first to fix some build warnings, and second to correct the screen equality behavior.

Before this PR, Screen.Equals(Screen) was delegating to base.Equals(object). I think it was meant to call the Equals(object) overridden in the derived PlatformScreen, but the base call make it non-virtual.

Basically, Equals(Screen) was always equivalent to ReferenceEquals() and thus different from Equals(object).

Equals(object) now calls Equals(Screen), which is overridden.
Ideally, we should make Screen, Screen.Equals(Screen) and Screen.GetHashCode() abstract in v12.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0052067-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

This PR adds missing overrides for Equals(object) and GetHashCode() on Screen.

These were removed from my refactoring PR, as implementation was relying on mutable properties, when IPlatformHandle wasn't available. Which was the case with Linux initially.
Now, since Linux backend provides a proper IPlatformHandle, these overloads make more sense.

Ideally, we should make Screen, Screen.Equals(Screen) and Screen.GetHashCode() abstract in v12.

Ideally, I would make Screen class sealed (or with an internal ctor). And only (i.e. - only) created by the windowing backend, always backed by the IPlatformHandle.