- rename client and session to start_client and start_session in
window_choose_data struct. also add TREE_OTHER define and reorder the
struct
- rename window_choose_ctx to window_choose_data_run
- don't pass a cmd_ctx into window_choose_create (will let it use a
different client later). instead take type, session, client
- add window_choose_data_free and use it to dispose of wcd rather than
each cmd-*.c doing it individually
- change so ref counting is done by wcd_add and wcd_free rather than
callers. this means 1 ref for each item but what of it :-)
- also add a ref to tree_session - not sure if this is needed?
- all the callbacks except choose-client and find-window are the same so
remove them and add window_choose_default_callback
- reorder/rename some other bits and pieces for tidyness
