Extract helper functions from calendar#show() and calendar#action()

- calendar#show() 560 -> 125 lines
  - s:init_table_layout(): table (dir==2) layout computation
  - s:build_month_string(): single month calendar string generation
  - s:merge_displays(): month display merging (horizontal/vertical/table)
  - s:open_calendar_window(): window creation and buffer setup
  - s:render_navi(): navigation label rendering

- calendar#action() 151 -> 34 lines
  - s:handle_navi(): navigation click processing
  - s:handle_day_action(): day resolution and action invocation

Author: mattn