Fix functionDescription()

Description

• fix wrong description is shown
• fix translation in constants

The problem is that _functions and _special_functions exist, but only one list with both concatenated: m_constantsDescription. Because of this when a special function is used (for example cell()) the index of cell will be used in this concatenated list and therefore the wrong description is used (cell 1 has index 0 in _special_functions, but in the concatenated it is 0 + _number_functions.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: #818 (closed)

When external dependencies are removed

• Reporting to ...