* `App.is_cursor_movement(key)` -- return `true` if `key` is a cursor movement
  key (arrow keys, page-up/down, home/end)
* `App.cmd_down()`, `App.ctrl_down`, `App.alt_down()`, `App.shift_down()` --
  predicates for different modifier keys.
* `App.any_modifier_down()` -- returns `true` if any of the modifier keys is
  currently pressed.