Add support for publishing multiple clipboard format on unix/x11

[LinqLover]

• Implement clearing of clipboard
• Different string formats are only offered when calling display_clipboardWriteWithType() with empty type - legacy interface used by old clipboard primitive. Users of ExtendedClipboardPlugin will have to provide different formats on the image side.
• Implement clipboard size/type requests (not sure why legacy clipboard even worked before)

Please review carefully before merging. I have little experience with C, and 80% of this patch came out of GitHub Copilot, even though I of course did my best to review and challenge it. Seems to work fine in practice on Ubuntu 24/GNOME though. Use System-ct.1490 (inbox) to test this in Squeak.

[Copilot]

Pull request overview

This PR adds support for publishing multiple clipboard formats on Unix/X11 systems by implementing a multi-format clipboard offer table and enhancing the clipboard API to support typed data. The implementation includes clipboard clearing functionality and proper size/type request handling.

Changes:

• Changed return type of clipboardWriteWithType from void to sqInt across all implementations to enable error reporting
• Implemented multi-format clipboard offer table using a linked list data structure to store multiple clipboard formats simultaneously
• Added clipboard clearing functionality via relinquishClipboard() and enhanced sqPasteboardClear()
• Refactored sendSelection() to serve clipboard data from the offer table instead of hardcoded format conversions

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 25 comments.

Show a summary per file

File	Description
platforms/unix/vm/sqUnixMain.c	Updated wrapper to return sqInt instead of void for clipboardWriteWithType
platforms/unix/vm/SqDisplay.h	Changed function pointer signature for clipboardWriteWithType to return sqInt
platforms/unix/vm-display-null/sqUnixDisplayNull.c	Updated stub implementation to return 0 instead of void
platforms/unix/vm-display-fbdev/sqUnixFBDev.c	Updated stub implementation to return 0 instead of void
platforms/unix/vm-display-X11/sqUnixX11.c	Core implementation: added SelectionOffer struct, offer table management functions, refactored clipboard write/read operations, and implemented clipboard clearing
platforms/unix/plugins/ClipboardExtendedPlugin/sqUnixExtendedClipboard.c	Implemented clipboard clearing and added formatCounter to track multi-format writes

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[Copilot]

Variable declarations after statements. In C89/C90, all variable declarations must appear at the beginning of a block before any statements. The declarations 'size_t capacity' and 'SelectionOffer *offer' appear inside an else if block after the capacity assignment statement. This may cause compilation errors with strict C89/C90 compilers. Move these variable declarations to the beginning of the function.

Suggested change

	size_t capacity= 3;
	SelectionOffer *offer;
	size_t capacity;
	SelectionOffer *offer;
	capacity= 3;

[Copilot]

Variable declarations after statements. In C89/C90, all variable declarations must appear at the beginning of a block before any statements. The declaration 'Atom *targets' appears after the for loop that increments capacity. This may cause compilation errors with strict C89/C90 compilers. Move the variable declaration to the beginning of the block.

Suggested change

	for (offer= offerTable; offer; offer= offer->next)
	capacity++;
	Atom *targets= (Atom *)malloc(capacity * sizeof(Atom));
	Atom *targets;
	for (offer= offerTable; offer; offer= offer->next)
	capacity++;
	targets= (Atom *)malloc(capacity * sizeof(Atom));

[Copilot]

Variable declarations after statements. In C89/C90, all variable declarations must appear at the beginning of a block before any statements. The declarations 'int i' and 'duplicate' appear inside the for loop after assignments to the targets array. This may cause compilation errors with strict C89/C90 compilers. Move these variable declarations to the beginning of the enclosing block.