Codechange: use \u to indicate unicode chars in strings (#8379)

With \x, we sometimes had to do the "" trick, as the length is not
predefined. With C++11 bringing \u to the specs, which has a preset
length, we no longer need the "" trick.

We set the strings to u8, to ensure all compilers use UTF-8 encoding
for the \u characters.

This was triggered by newer CLangs, which start to warn if you
use "" in the middle of a string, wondering if that was your
intention. It is a good question. And this is our answer :)

src/townname.cpp

@@ -505,7 +505,7 @@ static char *MakeFinnishTownName(char *buf, const char *last, uint32 seed)
 				strstr(orig, "A") != nullptr || strstr(orig, "O") != nullptr || strstr(orig, "U")  != nullptr) {
 			buf = strecpy(buf, "la", last);
 		} else {
-			buf = strecpy(buf, "l\xC3\xA4", last);
+			buf = strecpy(buf, u8"l\u00e4", last);
 		}
 		return buf;
 	}