fix loading infoelement during project load

Description

In 726aac97 a regression was introduced which causes skipping reading of the custom points in InfoElement::load(). This MR fixes the regression and has some minor cleanup and refactoring additionally.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: #817 (closed)

When external dependencies are removed

• Reporting to ...