Reaction Variations: Independent Layer/field-columns belonging to generic segments

[StarmanMartin]

• I am an developer

Currently, in the reaction variations only a single value of a generic segment. This is limiting, as users need to be able to modify all values independently.

Expected Behavior

Users should be able to edit multiple values without restriction

Current Behavior

Only one value can be edited at a time. Additional values are locked / not editable.

Use Case

This enables workflows where multiple properties of a segment need adjustment without jumping back and forth or saving repeatedly.

[JanCBrammer]

The following entities need to be represented in the variations table:

• generic segments
• generic layers
• generic fields

Their hierarchy is as follows: segments include layers, and layers include fields.
Their representation in the variations table is as follows: segments are column groups, layer/field-combinations are columns.

• users should be able to place layer/field-columns in any position in table; i.e., grouping under segment, as it's currently implemented, is not desirable (decouple grouping for segment groups in ag-grid with marryChildren=false)
• remove "layer: foo, field: bar" from title of layer/field-columns