Window header alignment does nothing

Looking through the source, there is no logic behind the alignment style for the window header. The only code related to this is on lines 5167 to 5170 and 5190 of `nuklear.h`:
```c
enum nk_style_header_align {
NK_HEADER_LEFT,
NK_HEADER_RIGHT
};
```
```c
struct nk_style_window_header {
...
/* properties */
enum nk_style_header_align align;
...
};
```
In the event this is only an issue with my backend, I am using the GLFW OpenGL 3 backend. `nuklear_glfw_gl3.h` had no code related to this alignment.

It would also be nice if you could center the header on top of a left or right alignment.

Start in nuklear.h around lines 5167-5170 and 5190, then trace where nk_style_window_header is rendered and its align field is consumed. Check nuklear_glfw_gl3.h only to confirm the behavior is not backend-specific. Done means the requested left and right alignment behavior works, with the desired centered-header behavior clarified and covered.

Written by the indexing model from the issue text.