Shift+Tab is an odd bird in XKB; it produces a shifted variant of tab,
ISO_Left_Tab, with Shift being a consumed modifier.
From a terminal perspective, Tab and ISO_Left_Tab are the same key,
with Shift+Tab generating ‘CSI Z’, and all other combos generating a
‘CSI 27;mods;9~’ escape.
Before, when we didn’t filter out consumed modifiers, we could simply
re-use the keymap for Tab when translating ISO_Left_Tab.
This is no longer possible, since shift has been filtered out. As a
result, Shift+Tab no longer generated ‘CSI Z’, but ‘\t’. I.e as if
shift wasn’t being pressed.
This patch adds a separate keymap table for ISO_Left_Tab. It’s MOD_ANY
entry corresponds to Shift+Tab, and emits ‘CSI Z’. All its other
entries *exclude* shift (since it has been consumed), *but*, the
encoded modifier (in the escape sequences) *include* shift.