const correctness in public API

[Xeverous]

This patch adds const to pointers to data types which aren't modified.

There are few minor issues remaining - I did not changed these but please inform me what's your opinion on these:

• nk_group_get_scroll - unlike with other getters, this getter can not use const context because the implementation modifies the context on one branch (the value is cached and created on-demand, which means getters need to be able to create it too)
• nk_find_value - the use of const would be questionable here, especially given the fact that the function returns non-const pointer. Perhaps it needs a second overload with both const input and const output?
• nk_combo_callback, nk_combobox_callback - shouldn't item_getter be changed from void(*)(void*, int, const char**) to const char*(*)(void*, int)?
• struct nk_font* nk_font_atlas_add_from_memory(struct nk_font_atlas *atlas, void *memory, nk_size size, float height, const struct nk_font_config *config); and likewise nk_font_atlas_add_compressed takes a non-const pointer to the memory but looking at the implementation this memory is never modified (actually copied).