use ph-status-busy only as a concrete file semaphore

e.g. set it to a file name in ph-project-close, then in ph-kill-file
hook do cl-return only if ph-status-busy == buffer-file-name.

* remove useless cl-lib 'require's
* add hook counters
add hook countes to tests
improve speed of (ph-project-switch-buffer)
add slow test
rewrite (ph-buffer-list)
add number of opened files in the menu