added support for combo from an array of struct

[gooosedev]

This PR adds a new combo function nk_combo_from_struct_array
the objective is to be able to make combos from an array of structs (the only requirement is for the struct to have a char* ptr to hold the name)

parameters are the same as for the nk_combo function, with the addition of the size of the struct and a stride to the
member to use for the combo entry.

example:

struct mystruct{
  float f;
  char* name;
  int i;
};
struct mystruct items [3]={
  {1.0, "foo", 1},
  {1.0, "foobar", 1},
  {1.0, "bar", 1},
};
int selected;
/* ... */
selected = nk_combo_from_struct_array(ctx, &items, 3, sizeof(struct mystruct), offsetof(struct mystruct, name), selected, 15, nk_vec2(x,y));

[RobLoach]

Thanks a lot. Would you mind adding a small example in the overview demo? It serves as a good space to test the code and make sure it's all still functional.

[gooosedev]

I added a small example as requested.
Please let me know if something else is needed.