Created API

Made it into an API so that developers can more easily adopt this typing method into their app.

Basically there is a single function exposed void tertiary_text_prompt( const char* title, TertiaryTextCallback callback, void* extra );

It opens a Tertiary Text window (with the title supplied at the top of the window) and allows input. Once the user holds select that input is send to the provided TertiaryTextCallback function along with the supplied 'extra' data.