[FIX] grid overlay: avoid bazillions of useless renders

[hokolomopo]

The grid overlay would trigger a re-render for each mouse move event. That's a bit much. A render is only useful when:

1. we're in dashboard mode
2. the hovered cell has changed

Task: 0

Description:

description of this task, what is implemented and why it is implemented that way.

Task: TASK_ID

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

[robodoo]