fix(linux): Migrate from /usr/lib/ibus to /usr/libexec

Current ibus versions use /usr/libexec in compliance with FHS 3.0.
Since the ibus picks up the location from the xml file in
`/usr/share/ibus/component`, this also works for older versions of
ibus. The bug report also mentions a `<setup>` tag. However, this
doesn't apply for Keyman because we don't have nor need a setup app.

Closes #4259.
Closes Debian bug 955230.