Allow GUI action functions that take a callback to use the function itself directly

[FabienLelaquais]

Description

A typical example of that limitation is the download() GUI action: its on_action parameter expects the name of the callback function where it would be far simpler to use the function value itself:

download(state, content=..., on_action="my_callback_name") is not really user-friendly compared to download(state, content=..., on_action=my_callback_name)

Acceptance Criteria

• Ensure new code is unit tested, and check code coverage is at least 90%
• Propagate any change on the demos and run all of them to ensure there is no breaking change
• Ensure any change is well documented