examples/web/async.py: make use of improved controls

[otaku42]

Falling back onto LegacyControls fixed the issue reported by @tiagocoutinho. However, that solution prevents leaveraging the improvements of the control API, which would allow stuff like:

• add a button to reset all controls to their default (using Controls method set_to_default)
• grey out inactive controls (using the is_writeable property of each control)
• use the config_name property of each control rather than their numerical id to reference controls in GET/PUT requests. which could help a bit to better understand what's going on in the log
• allow to save the current settings, and load and apply previously saved control settings (using the stuff in Support for configuration files tiagocoutinho/v4l2py#43)