[spreadsheet] faster cutting of cells

Description

When clearing selected cells, avoid iterations over indices that were already cleared for fully selected columns. These unneeded iterations are very costly, especially for bigger spreadsheets. Also, some other minor performance improvements relevant for this workflow.

For the project file attached in BUG 513574, this improvement reduces the time to do CTRL+X for one column from 15s to <<1s on i7-14700. On slower hardware the effect will be bigger. This also greatly reduces the memory consumption for this step since less undo-commands are put onto the undo stack.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: BUG 513574