Reaction Variations: Column configuration

[JanCBrammer]

Allow order and visibility of columns to be configured.

Enable users to select material-columns instead of automatically adding a column for each material in the Scheme tab.
Each material group contains a selection of materials from Scheme tab and users can pick which materials to include as columns. The same goes for the properties group.

Benefits of making users explicitly select columns instead of automatically adding and updating columns:

• No more need to keep state of Scheme and Variations tab consistent, which is error prone and needlessly bloats the implementation.
• Gives users more flexibility.