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.

from dash import Dash, dcc, html, Input, Output,callback

app = 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')
)
def update_output(value):
    return f'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?