imcontext: Replace the builtin sequences

imcontext: Replace the builtin sequences

When we find a Compose file, replace the builtin
sequences with the table we found. This matches the
semantics described in Compose(5), and makes it possible
to drop unwanted sequences from the builtin table.

It is slight change of behavior for users with existing
Compose files. To match the previous behavior, you have
to add

include "%L"

to your Compose file, to keep the builtin sequences in
addition to your own.