cwutils/easy rec: moving cw_easy_rec_poll() to module's public API
libcw/rec: fixing return type of some of poll functions
libcw/rec: renaming cw_easy_rec_data_t type and variables
libcw/rec: renaming two receiver states
libcw/rec: removing "is_error" member from cw_rec_data_t
libcw/rec: refactoring receiver's poll functions
Release checklist: indicating start of work on "easy receiver" issue
cwutils / easy receiver: changing type of cw_easy_rec_handle_keying_event()