New feature: Add user event in subscription Event

[jpercyasnet]

New feature: Events only have keyboard, mouse, touch and window. Need to create a user event (possibly a new iced command which provides string data) that can be captured by Subscription::Event. This would simplify the handling of temporary async functions passing info to the gui (i.e. updating progress bar). For example, start an async function to run a batch job editing a bunch of files and sending a user event showing its progress.