Don't write to package directory

[maleadt]

Gtk.jl currently, during __init__, writes to its own package directory:

Gtk.jl/src/Gtk.jl

Line 93 in 1924a81

mutable_artifacts_toml = joinpath(dirname(@__DIR__), "MutableArtifacts.toml")

This is not OK, as packages may be installed in a system-wide or otherwise read-only location. Gtk.jl should use Scratch.jl to get a directory to write temporary things to.

[tknopp]

#619

[maleadt]

Oh ok, sorry for the noise.