Add Z-order controls and dimension-based selection restrictions

[ThomasKroes]

Summary

This PR adds configurable Z ordering and dimension-based selection restrictions to the Scatterplot plugin.

Changes

Z ordering

• Add a dedicated Z-ordering settings section.
• Add three Z-ordering modes:
  • Insertion order
  • Dimension-based ordering
  • Randomized depth
• Add a dimension picker for dimension-based Z ordering.
• Forward the selected dimension values to the core point renderer.

Selection restrictions

• Add a canonical Selection Restriction action to the Selection settings.
• Allow users to restrict selection using:
  • A selected dimension
  • An inclusive lower limit
  • An inclusive upper limit
• Keep the selectable range visible but read-only in Z-ordering settings unless Z-order coupling is active.
• Add Restrict selection by Z-order dimension to couple the restriction to the active Z-order dimension.
• Synchronize the restriction state and selected dimension between the Selection and Z-ordering interfaces.

Selection behavior

• Maintain an exclusion mask in the scatterplot widget.
• Ensure excluded points cannot become effectively selected through:
  • Interactive area selection
  • Select all
  • Invert selection
  • Focus sampling
  • Selection-based sampling
  • Selection changes received through linked datasets
• Preserve the global linked-dataset selection while applying exclusions locally in the scatterplot.
• Update selection-related action labels and tooltips to report excluded points.

Heads-up display

• Add a Selection section showing:
  • The number of effectively selected points
  • The number of selectable points

Rendering

• Mark excluded points for the core renderer so they are displayed using the non-selectable-point style.
• The current style is an opaque neutral gray.
• The rendering style of non-selectable points will become customizable in a future update.

Dependency

This PR depends on the accompanying ManiVault core PR that adds:

• Configurable point Z ordering
• Per-point Z-order scalar values
• Non-selectable-point rendering support
• Correct heads-up display parent-child insertion