Visualization not compatible with POMDPs.jl ecosystem

Current visualization components aren't compatible with the rest of the POMDPs.jl ecosystem. In particular, this problem currently doesn't work with DisplaySimulator or GifSimulator without some modifications.

```julia
Warning: No implementation of POMDPTools.render(m::VDPTagPOMDP{Nothing}
│ mdp: VDPTagMDP{Nothing}
│ meas_cost: Float64 5.0
│ active_meas_std: Float64 0.1
│ meas_std: Float64 5.0
│ , step) found. Falling back to text default.
└ @ POMDPTools.ModelTools ~/.julia/packages/POMDPTools/7Rekv/src/ModelTools/visualization.jl:31
ERROR: MethodError: no method matching show(::IOStream, ::MIME{Symbol("image/png")}, ::String)
```

Start by reproducing the issue with DisplaySimulator and GifSimulator, then trace the visualization components involved in the missing POMDPTools.render implementation and the image/png display error. Done means the problem works with both simulators without modifications, warnings, or the reported MethodError.

Written by the indexing model from the issue text.