Update Tree Example

The Tree demo is broken. I tried following the deprication messages but I'm still running into problems.

╭─mcamp on Butler in ~/code/StippleDemos/BasicExamples/Tree on master🔥
╰─🚧 julia Tree.jl        
┌ Warning: @reactive! is deprecated, please replace use `@vars` instead.
│ 
│ In case of errors, please replace `@reactive!` by `@old_reactive!` and open an issue at
│ https://github.com/GenieFramework/Stipple.jl.
│ 
│ If you use `@old_reactive!`, make sure to call `accessmode_from_pattern!()`, because the internals for
│ accessmode have changed, e.g.
│ ```
│ model = init(MyDashboard) |> accessmode_from_pattern! |> handlers |> ui |> html
│ ```
└ @ Main ~/.config/julia/packages/Stipple/qnyBY/src/stipple/reactivity.jl:430
ERROR: LoadError: UndefVarError: register_mixin not defined
Stacktrace:
 [1] top-level scope
   @ ~/code/StippleDemos/BasicExamples/Tree/Tree.jl:39
in expression starting at /home/mcamp/code/StippleDemos/BasicExamples/Tree/Tree.jl:39
╭─mcamp on Butler in ~/code/StippleDemos/BasicExamples/Tree on master🔥
╰─🚧 vim Tree.jl        
╭─mcamp on Butler in ~/code/StippleDemos/BasicExamples/Tree on master🔥
╰─🚧 vim Tree.jl
╭─mcamp on Butler in ~/code/StippleDemos/BasicExamples/Tree on master🔥
╰─🚧 julia Tree.jl
ERROR: LoadError: type Bool has no field args
Stacktrace:
 [1] getproperty(x::Bool, f::Symbol)
   @ Base ./Base.jl:38
 [2] var"@var_storage"(__source__::LineNumberNode, __module__::Module, expr::Any, new_inputmode::Any)
   @ Stipple ~/.config/julia/packages/Stipple/qnyBY/src/stipple/reactivity.jl:258
 [3] var"@var_storage"(__source__::LineNumberNode, __module__::Module, expr::Any)
   @ Stipple ~/.config/julia/packages/Stipple/qnyBY/src/stipple/reactivity.jl:247
 [4] eval(m::Module, e::Any)
   @ Core ./boot.jl:368
 [5] var"@vars"(__source__::LineNumberNode, __module__::Module, expr::Any)
   @ Stipple.ReactiveTools ~/.config/julia/packages/Stipple/qnyBY/src/ReactiveTools.jl:151
in expression starting at /home/mcamp/.config/julia/packages/Stipple/qnyBY/src/ReactiveTools.jl:151
in expression starting at /home/mcamp/code/StippleDemos/BasicExamples/Tree/Tree.jl:27
in expression starting at /home/mcamp/code/StippleDemos/BasicExamples/Tree/Tree.jl:27

Start with BasicExamples/Tree/Tree.jl, especially the reported failures at lines 27 and 39, and run the demo to reproduce the errors. Compare the deprecated @reactive! guidance and the @vars and accessmode_from_pattern! examples in the report. Done means the Tree demo runs successfully without the reported UndefVarError and Bool field error.