Give a warning and a failure instead of an UnDefinedVarError if GTK is not loaded

I was very confused that i kept getting:
```
┌ Warning: Should we make this the new reference image?
└ @ VisualRegressionTests ~/.julia/packages/VisualRegressionTests/q0Rxt/src/imgcomp.jl:39
LayeredLayouts.jl: Error During Test at /Users/oxinabox/.julia/packages/VisualRegressionTests/q0Rxt/src/macros.jl:18
Test threw exception
Expression: test_images(testFilename, ref("1"), popup = true, tol = 0.02) |> success
UndefVarError: replace_refimg_dialog not defined
Stacktrace:
[1] test_images(::String, ::String; popup::Bool, newfn::String, kw::Base.Iterators.Pairs{Symbol,Float64,Tuple{Symbol},NamedTuple{(:tol,),Tuple{Float64}}}) at /Users/oxinabox/.julia/packages/VisualRegressionTests/q0Rxt/src/imgcomp.jl:40
```

when testing things.
I assumed there was a bug in the package.
But then i looked in the source and saw that Requires.jl is being used to include the file with that function defined in it, only if GTK was loaded.

We should be able to use `isdefined` to check if that function is defined.
and if it isn't then rather than having an undefined error,
say something like "Please load GTK to regenerate images"