Added:
* class FDPlane (`ncfdplane*`)
* class Subproc (`ncsubproc*`)
* NotCurses: get_inputready_fd (`notcurses_inputready_fd`)
* Plane: qrcode (`ncplane_qrcode`)
* class PlotBase: templated base class for Plot variations
* class PlotU: `uint64_t` instantiation of PlotBase (aliased to previous
`Plot` class for source compatibility), `ncuplot*`
* class PlotD: `double` instantiation of PlotBase, `ncdplot*`