Implement XDND: drag-and-drop support for Linux

[MrJul]

What does the pull request do?

This PR implements XDND for Avalonia's X11 platform, allowing windows to act as drag sources and drop targets.
In short, drag-and-drop now works across processes on Linux.

How was the solution implemented (if it's not obvious)?

This PR supersedes #19232, which had several identified core problems: threading issues and re-implementations of existing algorithms already provided by the clipboard.

Instead, this PR tries to share most of its code with the clipboard implementation, since the underlying concepts are similar.

Key points:

• Most of the clipboard logic has been moved to base Selection[...] classes, with Clipboard and DragDrop inheritors.
• Threading is straightforward: no dispatcher calls, no arbitrary timers (except for timeouts), no non-awaited tasks.
• Since drag-and-drop is synchronous, but X11 isn't, the SynchronousXEventWaiter type waits for specific matching events synchronously while reading from a selection, and puts back unused events onto the standard queue.
• All atoms used by Avalonia are now interned on startup, even if they don't exist.

Draft

This PR works. However, there are two points remaining:

• Touch/pen devices aren't properly captured. I focused on the mouse for the first draft because I didn't have a touchscreen readily available for testing. It's straightforward to add and will be implemented soon.
• Hardening against sources misbehaving and not sending a XdndFinished message in time, effectively hanging the drop operation.

Fixed issues

• Fixes [Known][Waiting] Drag and Drop files doesn't work on Linux #6085
• Supersedes Fix/drag and drop on linux #19232

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0063557-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]