Utilise transactions to keep preserve history

[phantomeye1931]

Thanks so much for publishing this script. It has made it really easy to keep my textures organised in 1 file and export them all at once. However, every time I would use it, it would fuck up my entire undo history. This PR is to resolve that.

In all 3 scripts, I utilised a transaction that bundles all edits together, and undoes them at the end. That way, the user's edit history is preserved, and it's restored to the state it was before running the script. This way, you can quickly go back and revert changes made before the export, without having to sit through hundreds of crops.

Before	After

The diffs show a lot of changed lines, but in reality I only added such wrappers:

-- Finally, perform everything, maintaining 1 transaction
app.transaction("Export Slices", function()
  -- ...
end)
-- Undo the transaction to revert history to before exporting
app.undo()