[ozone/wayland] Add capslock support

CL adds capslock support by extending the current modifiers
supported (shift, control, alt, et al).

It also makes it possible to mock XKB states, so that modifiers
can be unittest'ed.

BUG=578890

Change-Id: I524529fdb96939035e530d5a4ff80efc7681dd38
Reviewed-on: https://chromium-review.googlesource.com/867630
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529976}
4 files changed