Make client API for oak::server_t thread safe
Initialize shared spell checker during setup

The first time we ask for the shared spell checker it does some setup involving AppKit (creating the shared spelling panel?). While we already ensure that the spell checker is only called from the main queue, it seems this setup code must run on thread 0, which, when we are not in a Cocoa application, is not necessarily the thread used for the main queue.
Remove unused include