Change the whole library to not use any null-terminated strings

As discussed in #890

To make this change happen, it would be wise to change (not add new API calls) the whole library to not use any null-terminated strings. For this reason I'm currently a bit reluctant to approve this PR.

Would this involve removing APIs like nk_label and only having nk_text?

In this case yes as the only motivation to have nk_label was to support null-terminated strings wink