Update confirmation message onBeforeUnload

[juliandescottes]

The current message displayed when leaving the application can be confusing:

Users might click OK thinking that it will allow to save their changes. We should update the message so that it's clear that VALIDATING will lose all their unsaved progress.

Something like Your current sprite has unsaved changes. Are you sure you want to quit?

Note this is mostly relevant for desktop applications since most browsers now display a browser-specific message instead of what the website provides.