Improved performance of the worksheet preview

Description

Improved the performance of the worksheet preview.

• don't update the previews if the preview widget was hidden
• use the already existing pixmaps in the worksheet children instead of using the direct and expensive painting for all elements
• removed redundant changed() signals to reduce the number of worksheet preview recalculations

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport fix #830 (closed)