Resolve "Spreadsheet: Create function values: Wrong tool tip"

Description

Reworked the code to check the validness of the entries in FunctionValuesDialog and to show the proper tooltip text and to enabled/disable the ok-Button. The new code produces a better tooltip also for the cases when no formula was entered at all and when no valid column was selected for one of the variables.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport fix #792 (closed)

When external dependencies are removed

• Reporting to ...

Closes #792 (closed)