[FW][PERF] model: export lazily when leaving

[fw-bot]

Description:

Since 7bcff34, the model data is exported every time the spreadsheet is left.
Exporting the data can be slow though and it's actually not needed every time.

With this commit, the data is exported lazily, only when needed.

Task: : 4119536

review checklist

• feature is organized in plugin, or UI components
• support of duplicate sheet (deep copy)
• in model/core: ranges are Range object, and can be adapted (adaptRanges)
• in model/UI: ranges are strings (to show the user)
• undo-able commands (uses this.history.update)
• multiuser-able commands (has inverse commands and transformations where needed)
• new/updated/removed commands are documented
• exportable in excel
• translations (_t("qmsdf %s", abc))
• unit tested
• clean commented code
• track breaking changes
• doc is rebuild (npm run doc)
• status is correct in Odoo

Forward-Port-Of: #4848

[robodoo]

[fw-bot]

This PR targets saas-17.4 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port