Add Checkbox Element

[MLJWare]

Description

Implement a basic checkbox UI element.

• Simplistic, Material Design inspired look
• Two main states: "checked" and "unchecked"
  • Additional state: "indeterminate"

Intended Fields

• disabled (boolean): determines whether input should be processed.
• checked (boolean or "indeterminate"): indicating the state of the checkbox.
• on_change (old_state) callback invoked when state changes as a result of mouse input.