Bugfix: allow negative int values.

ConfigParser::ReadInteger can handle these, but ::ReadValue was never
calling ::ReadInteger if the leading character was a - (minus) sign.
This makes the input to xsettingsd consistent with the output of
dump_xsettings.

config_parser.cc

@@ -290,7 +290,7 @@ bool ConfigParser::ReadValue(Setting** setting_ptr) {
   char ch = stream_->GetChar();
   stream_->UngetChar(ch);
 
-  if (ch >= '0' && ch <= '9') {
+  if ((ch >= '0' && ch <= '9') || ch == '-') {
     int32_t value = 0;
     if (!ReadInteger(&value))
       return false;