This enables shifted alt-prefixed keys, such as `shift+alt+.` on
US standard becoming `M->`. To do this, we needed to fix a few bugs:
(1) translation mods should strip alt even if other mods are set
(2) AppKit translation event needs to construct new characters with
the translation mods.
(3) Alt-prefix handling in KeyEncoder needs to allow ASCII utf8
translations even for macOS.