This PR introduces allows the nav object to handle resizes. It also fixes a couple of bugs:

• The cursor can disappear when the nav height is reduced as a result of enabling drawbox:
  • Ensure the current directory has more files than what can be displayed in the UI
  • Press G to go to the very bottom
  • Enable drawbox
• The preview cache is cleared when setting ratios to the current value:
  • Add some kind of noticeable side effect to the previewer script (e.g. sleep, or write to a log file)
  • Run set ratios 1:2:3 (or whatever the current value of ratios is)