Create a new dropdown.
The values parameter should be an object where the keys are the display text and the values are the actual values.
Optional
label: string = ''The label text.
Optional
values: object = {}The values to display in the dropdown.
Optional
value: string = nullThe initial value of the dropdown.
Optional
callback: Function = nullThe callback function to call when the value is changed.
A class that represents a menu for selecting one of several values.