Option to make egui window visible on all workspaces

[mchaliadzinau]

It would be great to have ability to show window on all workspaces, even if current window is full-sized.

It is definitely possible for MacOS/Linux (for Linux it worked in nw.js app (X11, can't confirm it for Wayland)).
Electron allows it:

• for MacOS it was enabled with this PR: 🍎 Fix SetVisibleOnAllWorkspaces to show app over Fullscreen apps too electron/electron#11599