Our handling of left vs. right modifiers keys (shift, ctrl, etc.) was
largely not to spec. This patch adds explicit UIEvents::KeyCode values
for these keys, and updates the UI to match native key events to these
keys (as best as we are able).
(cherry picked from commit 4fcaeabe1a6acd9f4d5fd811a36f2e86748b2a72;
amended to make the rest of the system build and to keep `Mod_AltGr`
around in addition to adding it as a key)
Co-authored-by: Nico Weber <thakis@chromium.org>
965e1baa8c