gtksourceview: Provide a better default style #44

Job	Run time
build / build (lmde6, linuxmintd/lmde6-amd64, LMDE 6, false) / LMDE 6	2m 42s
build / build (mint22, linuxmintd/mint22-amd64, Mint 22, true) / Mint 22	2m 47s
	5m 29s

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