Improve the update mechanism for ui.plotly

[falkoschindler]

This PR tries to solve #4186 by calling the update() method in JavaScript explicitly rather than within the updated hook. It seems to resolve the problem described with the following code snippet:

ui.label('1. Manually zoom the plot')
ui.button('2. Update the label', on_click=lambda: label.set_text(label.text + '.'))
ui.button('3. Update the plot', on_click=lambda: plot.update())
ui.label('4. --> The plot resets unexpectedly')

label = ui.label('...')
plot = ui.plotly({'layout': {'uirevision': 'constant'}})

Tests are passing and demos seem to work like before.