unstable: add xcursor-configuration

Clients are responsible for loading and setting cursors. For this purpose they often use XCursor images via the libwayland-cursor library. However clients have no way to know the user's preferred theme and size. This results in inconsistent cursors across clients (e.g. cursor growing bigger when entering a surface).

This protocol fixes this issue by sending the user's XCursor preferences to the client. It also allows for per-device cursor configuration.

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Previously submitted on the mailing list: https://patchwork.freedesktop.org/series/50948/

Implementations:

• Qt: https://codereview.qt-project.org/c/qt/qtwayland/+/247935
• wlroots: https://github.com/swaywm/wlroots/pull/1324
• libwayland-cursor: https://patchwork.freedesktop.org/series/51322/