Cleanup
Lua: Move api completer after inbuffer

For a sub match the api completions will be authoritive, and
for global matches the inbuffer completer yields better results.
Extend inbuffer completer to cover multiple buffers

The exact nr of buffers are controlled by
config.inbuffer_completion_max_buffers (default 4). Whether to consider
only buffers with the same mode or not is controlled by
config.inbuffer_completion_same_mode_only.