Add a Rerun app icon for Linux

For the task-bar, when alt-tabbing, etc.

* [ ] Linux
  - https://github.com/emilk/egui/issues/3992
* [x] Mac
  - https://github.com/rerun-io/rerun/pull/1184 
* [x] Windows
  - https://github.com/rerun-io/rerun/pull/1222

Related:
* https://github.com/emilk/egui/issues/920
* https://docs.rs/eframe/latest/eframe/struct.NativeOptions.html#structfield.icon_data