add debounce to Dropdown

[AnnMarieW]

closes#3620

This PR adds an optional debounce (boolean) prop to the Dropdown

When debounce=True, changes to selected values will be sent back to the Dash server only when dropdown menu closes.

fromdashimportDash, dcc, html, Input, Output,callbackapp=Dash()
app.layout=html.Div([
html.Div(id='dd-output-container'),
dcc.Dropdown(
['NYC', 'MTL', 'SF'],
'NYC', id='demo-dropdown',
debounce=True,
closeOnSelect=False,
multi=True),
])
@callback(Output('dd-output-container', 'children'),Input('demo-dropdown', 'value'))defupdate_output(value):
returnf'You have selected {value}'if__name__=='__main__':
app.run(debug=True)

[AnnMarieW]

The two failed tests in dcc-312 are for date pickers which is unrelated to this PR. Flakey tests?

[T4rk1n]

💃