Road to 1.0

I'd like to move the whole JuliaImages stack to version numbers >= 1.0 (see https://github.com/JuliaImages/Images.jl/issues/825). Let's use this issue to collect plans for breaking changes in the near future.

Breaking changes include things like renamings, returning a different value than we used to (e.g., #129 might qualify, though rounding errors are a big ambiguous), or removing externally-visible functionality. Bug fixes, new features, performance improvements, etc., do not count as breaking, though of course they are very welcome.