Add support for XDG base directory spec #516

Job	Run time
determine_asy_version	10s
misc-sanity-checks / check-if-non-gui-py-files-changed	11s
linux-sanity / build-and-test-asy-compact-zero	24m 18s
build-asy-windows / configure-windows-msvc-x64	25m 53s
build-asy-linux / configure-linux-release-x64	18m 10s
build-gui / build-gui-linux	23s
build-gui / build-gui-windows	1m 47s
misc-sanity-checks / lint-python-files	0s
build-asy-linux / build-linux-release-x64	3m 37s
build-asy-linux / linux-asy-docgen	2m 22s
build-asy-linux / package-asymptote-artifacts	10s
test-asy-linux / test-x64-asy-tests	1m 41s
test-asy-linux / test-x64-cxxtests	30s
build-asy-windows / build-windows-msvc-x64	8m 19s
package-asy-install-file / package-asy-installer	49s
test-asy-windows / test-windows-msvc-x64-cxxtests	33s
test-asy-windows / test-windows-msvc-x64-asy-tests	4m 13s
precheck-pass	3s
	1h 33m 9s

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