Extract Curses differences and adaptation sections, and
move them to doc/CURSES.md. Reference this new document
in README.md. We now have sufficiently few sections that
we can dispense with the ToC. Do so.
Eliminate NCDIRECT_OPTION_NO_READLINE (it was only introduced in
Notcurses 2.1.6). Add a new function, ncdirect_readline(). Upon
first call to this function, initialize libreadline. Destroy
libreadline in ncdirect_stop() iff we initialized it. Add
hilodirect guessing game PoC from #1325. Rename
notcurses_directmode.3 to notcurses_direct.3. Closes#1326.
Extract `libnotcurses-core` from `libnotcurses`. The former contains everything except multimedia code. The latter contains multimedia stuff (a wrapper around FFmpeg or OIIO). If built with `-DUSE_MULTIMEDIA=none`, there will not be any `libnotcurses.so` generated. `libnotcurses.so` uses library constructors/destructors to insert its implementation into the `ncvisual` stack at runtime. Users linking `-lnotcurses` will get the full implementation; users linking `-lnotcurses-core` only will get the stack less multimedia code.
The upshot of this is that someone can compile/install only `libnotcurses-core`, and a program linked against it will work just fine. This eliminates the need to install the full (large) dependency stack of the multimedia code unless necessary. This will hopefully be useful for e.g. installers etc. Closes#339.