[#2] Redraw on terminal window resize (SIGWINCH).

Add `Display::Screen#term_height`,`#term_width` to report terminal lines
and columns instead of always using `Curses.lines`,`.cols`, since the
latter do not update after resize. Add `Display::Screen#missized?` to aid
detection of a window which is still resizing or missized otherwise.

Add `App#restart` for restarting an app, extending `#refresh_loop` to do
the actual restart.

Trap `SIGWINCH` calling `App#restart` in a similar way to `SIGINT`.