Initial implementation of display of R2L paragraphs in GUI sessions.

Initial implementation of display of R2L paragraphs in GUI sessions.

 xdisp.c [HAVE_WINDOW_SYSTEM]: Add prototype for
 append_stretch_glyph.
 (set_cursor_from_row) <cursor_x>: Remove unused variable.  Fix
 off-by-one error in computing x at end of text in the row.
 (extend_face_to_end_of_line): If the row is reversed, prepend a
 stretch glyph whose width is such that the rightmost glyph will be
 drawn at the right margin of the window.
 (append_stretch_glyph): In reversed row, prepend the glyph rather
 than append it.  Set resolved_level and bidi_type of the glyph.