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* `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.