fictional_handle -Wunused-variable

Curious if `fictional_handle` can be removed or avoided? Building without `STB_TRUETYPE_IMPLEMENTATION` on gcc (7.5.0) with `-Wunused-variable` complains that `fictional_handle` is unused.

https://github.com/Immediate-Mode-UI/Nuklear/blob/9f0bca461b028c1f8b638beeba1859045ebe1ac3/src/nuklear.h#L5473

It also seems strange not to have an `nk_` prefix for a global that appears outside an `NK_IMPLEMENTATION` block.

This makes development builds with `-Werror` harder.

Inspect src/nuklear.h around line 5473 and compare builds with and without STB_TRUETYPE_IMPLEMENTATION using GCC's -Wunused-variable. The issue is done when fictional_handle no longer produces an unused-variable warning in the affected configuration without introducing a new warning or breaking the header-only build.

Written by the indexing model from the issue text.