Made the ttk::treeview cell padding scaling-aware. #440

Job	Run time
build (gcc)	1m 49s
test (gcc)	3m 47s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 54s
test (gcc, --disable-xft)	3m 51s
build (gcc, --disable-xft)	1m 40s
test (gcc, --enable-symbols)	3m 21s
build (gcc, --disable-xss)	1m 40s
build (gcc, --enable-symbols)	1m 15s
build (clang)	1m 59s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	2m 3s
build (clang, --disable-xft)	1m 58s
build (clang, --disable-xss)	1m 47s
build (clang, --enable-symbols)	1m 35s
	28m 39s

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