Preferences: Work around issue with parsing unsigned integers
As we handle unsigned integers the same way as signed integers, large unsigned values (notably RGBA values) overflow the signed type. The issue became apparent when we switched from atoi to strtol in 0e92e8e5 While the former has undefined behavior for out of range numbers it luckily seemed to "just" overflow as expected on supported OSs. The latter however returns LONG_MAX (which for a 32-bit value is 0x7fffffff and turned out to show turquoise in unexpected places, e.g. the default path highlighting color in node tool). Avoid this by catching the error and try to parse as unsigned type in this case (which will still overflow, but seems to have worked as expected so far).
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