Add GdkArray

Add GdkArray

This is a scary idea where you #define a bunch of preprocessor values
and then #include "gdkarrayimpl.c" and end up with a dynamic array for
that data type.

See https://en.wikipedia.org/wiki/X_Macro for what's going on.

What are the advantages over using GArray or GPtrArray?

 * It's typesafe
   Because it works like C++ templates, we can use the actual type of
   the object instead of having to use gpointer.

 * It's one less indirection
   instead of 2 indirections via self->array->data, this array is
   embedded, so self->array is the actual data, and just one indirection
   away. This is pretty irrelevant in general, but can be very noticable
   in tight loops.

 * It's all inline
   Because the whole API is defined as static inline functions, the
   compiler has full access to everything and can (and does) optimize
   out unnecessary calls, thereby speeding up some operations quite
   significantly, when full optimizations are enabled.

 * It has more features
   In particular preallocation allows for avoiding malloc() calls, which
   can again speed up tight loops a lot.
   But there's also splice(), which is very useful when used with
   listmodels.