RFC: Allow recursive merging of Attributes

[mkborregaard]

Allows:

a = Theme(color = :red, axis = Theme(frames = ((true, false), (true, false))));
b = Theme(color = :blue, foo = false, axis = Theme(frames = ((false, false), (false, false))));
c = Theme(foo = true, scatter = Theme(color = :red));
merge(a,b,c).attributes
# Dict{Symbol,Observables.Observable} with 4 entries:
#   :color   => Observable{Any}("ob_05", :blue, Any[])
#   :scatter => Observable{Any}("ob_13", Attributes(Dict{Symbol,Observables.Observable}(:color=>Observable{Any}("ob_08", :red, Any[]))), Any[])
#   :axis    => Observable{Any}("ob_03", Attributes(Dict{Symbol,Observables.Observable}(:frames=>Observable{Any}("ob_04", ((false, false), (false, false)), Any…
#   :foo     => Observable{Any}("ob_09", true, Any[])

[mkborregaard]

the name is a joke of course and should be changed before merging

[mkborregaard]

You're happy with the function name? :-D

[SimonDanisch]

Can still be changed, and I wanted to progress ;)