Create `property` for overlay visible

[jo-mueller]

From an architectural point of view it would be handy to have a property overlay_visible or something that makes triggering the visible/invisible icon of the biaplotter a bit easier. It would make tracking the state of the visibility a bit easier.

cc @zoccoler @stefanhahmann

[zoccoler]

This should be solved after #49 with the property show_color_overlay.

Also, IMO v0.3.0 is ready to release, I just didn't have the time to write the changes in the release statement.