UTF-8 support for para_gui.

UTF-8 support for para_gui.

This adds two public helper functions to string.c which operate on
multibyte strings if the character encoding used in the selected
locale is UTF-8.

The first new helper, strwidth(), computes the width of the UTF-8
string while skip_cells() determines the number of bytes the given
multibyte string must be advanced in order to skip the given number
of cells.

para_gui is changed to use the new functions to properly display
UTF-8 encoded data in its top and bottom windows.