curses UI: properly truncate q-register and file names

 * format_str() will now automatically right-truncate strings (with
   formatted control characters) exceeding the window width (or provided
   maximum width of formatted characters).
   This means that Q-Register names in the info line are now properly
   truncated.
 * introduced format_filename() which takes care of canonicalizing
   control characters in a given filename and truncate the filename
   if necessary. Filenames are currently left-truncated since the file
   name proper and the directory containing it is more significant
   than the directory names closer to the root.
   On Windows, we make sure that the drive letter is not truncated.
   The truncation rules may have to be further tweaked later.
   This feature properly truncated file names in the info line and
   in the auto-completion popup.

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