Prevent leaflet from resetting center and zoom when browser resizes

[falkoschindler]

This PR tries to solve #4182 by changing the update mechanism for center and zoom yet again.

As it turned out, Vue's updated() lifecycle hook is not only called when a prop is updated from the server, but also when the window resizes (vertically). In this case the "center" and "zoom" props can be outdated if the user navigated the map manually. But the props are used to call setView, causing the map to reset to the last location sent from the server.

Various test setups from this issue as well as previous issues seem to work just right:

l = ui.leaflet(zoom=5)
ui.button('set center', on_click=lambda: l.set_center((48.1373, 11.5755)))

@ui.page('/')
async def page():
    m = ui.leaflet(zoom=5)
    central_park = m.generic_layer(name='polygon', args=[[
        (40.767809, -73.981249),
        (40.800273, -73.958291),
        (40.797011, -73.949683),
        (40.764704, -73.973741),
    ]])
    await m.initialized()
    bounds = await central_park.run_method('getBounds')
    m.run_map_method('fitBounds', [[bounds['_southWest'], bounds['_northEast']]])

m = ui.leaflet(center=(48.1, 11.6), zoom=10)
m.on('map-moveend', lambda e: print(e.args['center'], e.args['zoom']))
ui.button('Berlin', on_click=lambda: m.run_map_method('flyTo', [52.5, 13.4], 9, {'duration': 1.0}))