Legend controls: second pass

### User Story Description

The MVP list contained the legend panel. That panel for the MVP had minimal functionality. New functionalities need to be added, in particular:

- control orientation of the legend (horizontal / vertical)
- color palette (select from a list of palettes. Note: usually tools show both the name and a schema of the palette)
- control number of palette levels. You can control the number of levels, value at each level and exact color
- type of interpolation (linear vs log) between palette levels
- color to use for undefined value. The user should set from the color wheel or RGB
- pick the interpolation type for the palette: continuous, banded, constant

### Acceptance Criteria

Launch a VISOR session. Color one part by a variable. In the legends panel, all the new functionalities are available and work.

Save a state file. Launch a new VISOR session. Load the state file. Verify that all the legend advanced options are stored correctly.

### Additional Links

_No response_

### Requires new or update to third party ? If "yes" add the name(s) and the version(s) of the Third Party Software.

No

### Name and Version for Third Parties

_No response_

Launch a VISOR session and inspect the existing legends panel and state-file workflow first. Implementing the requested orientation, palette, levels, interpolation, undefined-value color, and persistence options is done when they work for a colored variable and reload correctly from a saved state file.

Scritto dal modello di indicizzazione a partire dal testo della issue.