Rewrite ImageView based on GtkReactive

[timholy]

Here's the NEWS entry, pasted for convenience (also, you can see the new README here):

New in 0.5

ImageView has been rewritten from scratch. Effort was made to maintain
backward compatibility where possible.

Breaking changes

• The return value of imshow has changed; it is now a Dict that
  stores Gtk widgets, Reactive signals, etc.

Major features

• This package now uses Gtk. Rendering is considerably faster in some
  cases, and the package is faster to load due to precompilation.

• Navigation and zoom region are controlled by GtkReactive/Reactive
  signals, allowing one to more easily extract this information for
  reuse elsewhere. Examples are shown in the README.

• A new contrast GUI is independent of any plotting package, leading
  to faster loading and faster time-to-first-plot.

• One can now display objects that are not subtypes of
  AbstractArray. See test/cone.jl for a demonstration.

Deprecations

• canvasgrid now returns more arguments; a deprecation warning
  encourages transitioning to the new syntax

• pixelspacing is deprecated as a keyword (use an AxisArray instead)

• The xy keyword has become axes, and it takes dimension integers
  or Symbols (if the image is an AxisArray)

<End of NEWS entry>

For me a major point of the rewrite was the ability to interactively "couple" images together, for example to show an original and segmented image side-by-side, and be able to zoom into the same region on both of them without any effort on the part of the user.

There aren't that many watchers of this package, so to give folks a fair chance to complain let me ping some people who have been active developers for the broader JuliaImages ecosystem: @Evizero, @bjarthur, @mronian, @tejus-gupta, @annimesh2809, @SimonDanisch, @ihnorton, @tkelman, @juliohm, @Cody-G, @tknopp, @mdhe1248.

[Evizero]

From reading the readme this looks really amazing. I'll be sure to try it out soon

[bjarthur]

woa, this seems so much more complicated than ImageInTerminal.jl 😄

[rsrock]

Nice work! Can't wait to give this a spin.

[timholy]

woa, this seems so much more complicated than ImageInTerminal.jl 😄

It's really hard to compete with ImageInTerminal's elegance. 3d/4d is my niche!

[timholy]

This is working well in my testing. I'll merge over the weekend barring objections.

[rsrock]

I did some quick testing, and it was very nice. I did run into an issue with the contrast control on macOS. Unfortunately, I don't have time to dig into it at the moment to give you a report-- perhaps in a few weeks.

[timholy]

Thanks for testing. No problem about the report, presumably someone else will complain and hopefully that will be enough to figure out the problem.

I want to come up with an easier API for linking two axes together, so there will be some additional changes later. But I don't think that needs to hold this up.