feat(linux): improve logging in ibus-engine-keyman

Currently we mainly use `g_message` (and a few `g_debug`) calls for logging in ibus-keyman. The output of `g_message` is usually not visible unless you explicitly start `ibus-engine-keyman`. However, it would be better if the user would have to explicitly request logging.

We should change most `g_message` to `g_debug` and might want to change the existing `g_debug` to use a separate log domain[1] so that we can distinguish between different levels of logging.

We could define our own log domain: `-DG_LOG_DOMAIN=keyman` (and use a separate domain for the existing `g_debug` calls, e.g. `keyman-debug`).

We might want to add the parameters `--verbose` (which would output from existing `g_message`) and `--debug` (which would additionally output from existing `g_debug`) to `ibus-engine-keyman`. The program then has to set (or append) the `G_MESSAGES_DEBUG` environment variable internally.

The user can then enable logging by either starting `ibus-engine-keyman` with `--verbose` or `--debug`, or by setting the environment variable `G_MESSAGES_DEBUG` to `keyman` or `keyman-debug`, e.g. `export G_MESSAGES_DEBUG=keyman,keyman-debug`.

See:
- [1] https://stackoverflow.com/a/69306792
- https://docs.gtk.org/glib/running.html
- https://docs.gtk.org/glib/logging.html

Related: #12703