Don't use std::index_sequence, as this has some advantages:
- This way doesn't require using another helper function, which is
burdensome here and would be even more so if we had to do it for all
the other wx vararg functions too.
- We don't need to require C++14 and can keep supporting even g++ 4.8.
The sole drawback is that we cannot pass the index of the argument to
wxArgNormalizer ctor any longer, which means that we can't validate it,
but this is addressed by validating the entire format string at once in
the new Validate() member function, which is also more efficient than
the old way for the format strings with more than one format specifier,
as they only need to be parsed once now, instead of having to do it for
each format specifier separately when GetArgumentType() is called.
This also means that we can't handle char arguments differently
depending on whether they are used with "%c" or "%d" format specifier.
This is a backwards-incompatible change, but should affect very few use
cases, so it seems to be worth breaking it to get the above benefits.
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