[worksheet] properly update the z-value of the worksheet elements when moving

Description

Properly update the z-value of the worksheet elements when moving the elements in the child hierarchy.

Earlier, the move was implemented via removeChild and addChild at the new position and the z-values were updated when the child was added. With the new and more efficient implementation of the move, we don't remove and add anymore and need to adjust the z-values on move now additionally.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: #942 (closed)

When external dependencies are removed

• Reporting to ...