Clear the keyed-archiver deprecation warnings - #584

Move the drag-and-drop pasteboard payloads onto the secure-coding keyed
archiver API that replaced the entry points deprecated in 10.14, and
handle the failure the replacement reports rather than letting it fall-through.

• Archive the file-change and ref drags requiring secure coding
• Decode them through unarchivedObjectOfClass(es):fromData:error:
• Name NSNumber alongside NSArray so the ref payload decodes fully
• Refuse the drag when encoding or decoding fails

The last point matters beyond the warnings.
The previous API raised on malformed data, while the replacement returns nil, and
neither drop handler was written for a nil payload.
On the file-change side that would have reached objectsAtIndexes: with nil - and crashed;
on the ref side it was quieter and worse, since messaging nil yields an index of 0 and
the drop would have moved the wrong ref.
Both paths now log and return NO.

Warnings go from 69 to 65 in GitX's own sources.
The remainder come from the vendored ObjectiveGit and libgit2 headers and
are not addressable from this repository.

Test plan

• xcodebuild test -only-testing:GitXTests passes 39/39 - unchanged
• Clean build drops exactly the four intended warnings and adds none,
  (confirmed by diffing the deduplicated warning sets)
• Exercised in a running build against a scratch repository:
  dragging a file from unstaged to staged and back,
  dragging a multiple-file selection, and
  dragging a branch label onto another commit,
  (which the reflog records as the expected ref update)
• The round trip was also checked directly for both payload shapes,
  including the index set class a real table-view supplies, and a
  wrong allowed class was confirmed to return nil with an error
  rather than a partly-decoded object