Remove redundant show that causes eventloop to remain active

[IanButterworth]

Opening this to remind myself.

If JuliaGraphics/Gtk.jl#607 lands, removing this show will mean the Gtk event loop goes to sleep when windows are closed.

With and without that line tests pass locally and I can't tell a functional difference, though I'm not sure I know what to look for.
I suspect it's redundant as the widget gets shown by another showall etc.

I also moved over to the new test/Project.toml format given the lowest julia version supported is 1.6

[codecov]

Codecov Report

Merging #259 (8a0451a) into master (23c3808) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
- Coverage   66.94%   66.91%   -0.04%     
==========================================
  Files           7        7              
  Lines         944      943       -1     
==========================================
- Hits          632      631       -1     
  Misses        312      312              

Impacted Files	Coverage Δ
src/ImageView.jl	89.15% <ø> (-0.04%)	⬇️

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[IanButterworth]

Fixed upstream. See JuliaGraphics/Gtk.jl#607 (comment)