help: Use color_t::from_hex_string() instead of from_argb_bytes()

Colours obtained from help markup until now have been generated using
from_argb_bytes(), even though the caller ensures they are 7 characters
long and thus in #RRGGBB hex notation. When porting the help text render
code to pango_text, this resulted in text with colour set by markup
being rendered invisible with an alpha value of 0.

Unless we decide at a later point to support alpha values in help
markup, we want alpha to always be 255.

src/help/help_impl.cpp

@@ -1338,7 +1338,7 @@ color_t string_to_color(const std::string &cmp_str)
 	}
 	// a #rrggbb color in pango format.
 	if (*cmp_str.c_str() == '#' && cmp_str.size() == 7) {
-		return color_t::from_argb_bytes(strtoul(cmp_str.c_str() + 1, nullptr, 16));
+		return color_t::from_hex_string(cmp_str.substr(1));
 	}
 	return font::NORMAL_COLOR;
 }