update Dropdown programmatically

[bjarthur]

previously, you could update option_index programmatically, but the display wouldn't change. now it does.

julia> using Bonito, Observables

julia> options = Observable(["a","b","c"])
Observable(["a", "b", "c"])

julia> V = []
Any[]

julia> app = App() do
           dd = Dropdown(options[])
           push!(V, (; dd))
           return dd
       end

julia> V[1].dd.option_index[]
1

julia> V[1].dd.option_index[]=2
2

[bjarthur]

the second commit in the PR will also cause the display to update when changing the options list programmatically. works fine when the current index is less than the length of the new options, but throws this error otherwise:

julia> V[1].dd.options[] = ["1","2"]
ERROR: BoundsError: attempt to access 2-element Vector{Any} at index [3]

looking into how to fix this now. suggestions welcome!

[bjarthur]

@SimonDanisch thoughts? i been using this PR in a dev branch and it seems fine so long as the developer makes sure the index does not exceed the length of the options.

[SimonDanisch]

Merged in https://github.com/SimonDanisch/Bonito.jl/pull/225