[FEAT] Add an invisibility marker for tokens marked as 'Is invisible'

[rexy712]

When setting a shape to be invisible, there is no indication outside of the properties menu for the GM to know that the shape is invisible to the players. This has led me to forget to make tokens visible from time to time and everyone being confused for a minute.

Describe the solution you'd like
Much like the 'defeated' state has a visible indicator (a big red X on the token), I think an indicator for invisibility would be nice. Below is an example I quickly put together. Left is normal image, right is with some invisibility marker.

There are plenty of different icons or overlays that could be used to convey invisibility. A full token, semi-transparent overlay of an invisibility symbol is also an option, this would match with the full token red X for defeated.

[dthv]

Second!

For the sake of completeness: An indicator for flying tokens has been proposed in #563 while nametags and/or status on mouseover was proposed/discussed in #657