do not show possible completions for hidden files and directories

 * added platform-dependant file_is_visible() function
Curses: fixed formatting of popup windows

 * simplified code
 * fixed spurious empty lines in the popup which truncated file names/tokens
   that would otherwise be displayed
 * fixed memleak when freeing the popup entry list
Curses: support cycling through long lists of possible auto-completions and optimized screen refreshing/redrawing

 * pressing e.g. TAB when the popup is showing a list of auto-completions
   will show the next page, eventually beginning at the first one again.
 * do not redraw curses windows in the UI methods directly. this resulted
   in flickering during command-line editing macros and ordinary macro calls
   because the physical screen was updated immediately.
   Instead, window refreshing and updated is done centrally in
   event_loop_iter() only after a key has been processed.
   Also we use wnoutrefresh() and doupdate() to send as little to the
   terminal (emulator) as possible.
support filename auto completions with forward-slash directory separators on Windows

 * this is actually UNTESTED on Windows