Use `Scratch` instead of `MutableArtifacts.toml`

[staticfloat]

Significantly simplify gdk-pixbuf querying by using a scratch space to hold mutable state. Also try to avoid needing to copy around the pixbuf loaders; instead opting for a more dynamic approach where we invoke gdk-pixbuf-query-loaders multiple times, concatenating the output appropriately.

[codecov[bot]]

Codecov Report

Base: 60.61% // Head: 63.84% // Increases project coverage by +3.22% :tada:

Coverage data is based on head (01d8736) compared to base (530da45). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
+ Coverage   60.61%   63.84%   +3.22%     
==========================================
  Files          32       32              
  Lines        2717     3214     +497     
==========================================
+ Hits         1647     2052     +405     
- Misses       1070     1162      +92     

Impacted Files	Coverage Δ
src/Gtk.jl	91.46% <100.00%> (-0.61%)	:arrow_down:
src/gtktypes.jl	33.33% <0.00%> (-40.01%)	:arrow_down:
src/GLib/GLib.jl	58.33% <0.00%> (-18.94%)	:arrow_down:
src/GLib/gtype.jl	63.13% <0.00%> (-12.90%)	:arrow_down:
src/boxes.jl	0.00% <0.00%> (-4.50%)	:arrow_down:
src/GLib/gvalues.jl	83.87% <0.00%> (-4.05%)	:arrow_down:
src/selectors.jl	24.22% <0.00%> (-1.01%)	:arrow_down:
src/misc.jl	0.00% <0.00%> (ø)
src/menus.jl	80.00% <0.00%> (ø)
... and 10 more

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[staticfloat]

@tknopp what do you think about bumping the minimum required version to 1.6? That's best for Scratch usage anyway, since that's the point at which Pkg.gc() knows how to clean these things up.

[tknopp]

Requiring 1.6 is fine for me. Give me a ping once this is ready an passing tests.

[maleadt]

+1 on this change; Gtk.jl modifying it's own directory is currently breaking PkgEval for packages that depend on Gtk.jl.