I'm wrapping this library into C++ (mostly for ease of use, reduced verbosity and increased code productivity; might release the code at some point) and I have noticed that const is rarely used.

Now, I know this is an immediate mode library and thus any widget-producing function can not have const nk_context* but many getter-only functions such as xxx_get_scroll for sure can take const context.

There are also minor issues in some specific functions, e.g.:

• NK_STRTOD has extra const qualifier that blocks using standard library #700
• nk_combo, nk_combobox and all related functions should take const char *const *, not const char ** as neither the strings nor the array is modified

I know very little internals of the library, but this issue is purely on the abstraction level so I should be able to make a PR. This would not be a breaking change. Any thoughts?