escape spaces in labels

refstyle doesn't work with these. See
https://marc.info/?l=lyx-users&m=169926958114185&w=2

src/support/lstrings.cpp

@@ -1230,7 +1230,8 @@ docstring const escape(docstring const & lab)
 	for (char_type const c : lab) {
 		if (c >= 128 || c == '=' || c == '%' || c == '#' || c == '$'
 		    || c == '}' || c == '{' || c == ']' || c == '[' || c == '&'
-		    || c == '\\') {
+		    || c == '\\' || c == ' ') // refstyle doesn't support spaces
+		{
 			// Although char_type is a 32 bit type we know that
 			// UCS4 occupies only 21 bits, so we don't need to
 			// encode bigger values. Test for 2^24 because we