chore(linux): Rename internal method to `commit_string`

The previous method name `ibus_keyman_engine_commit_string`
suggests that this method is overridden from ibus-engine class.
However, while it is true that ibus-engine has a
`ibus_engine_commit_string` method it can't be overridden. This
change renames the method to make it clearer that this is an internal
method that won't be called by ibus.