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.
