lib: window size gets read at init
lib: prefer shorter error constant name
lib: reorder the functions to match header
lib: forgot to rename error constant
lib: interface changes

- Get client code to deal with terminal width/height changes.
- Consider multithreaded code using the library.
lib: redone interface to library for regions

- Interface for non-overlapping regions which support automatic sizing
  based on terminal size and anchoring for dynamic placement.
- Ensure that editing different regions in the terminal is thread safe.
- jx_begin and jx_end functions to lock regions.
- Renamed jx_init to jx_initialise (with alias jx_initialize for
  American English speakers).
- Renamed jx_end to jx_terminate to disambiguate from the jx_begin and
  jx_end block.