Add an intermediary face for mode lines: `mode-line-active'

Add an intermediary face for mode lines: `mode-line-active'

* doc/emacs/display.texi (Standard Faces): Document the new face.

* lisp/faces.el (mode-line-active): New face.
(mode-line): Don't inherit from vaiable-pitch.

* src/xfaces.c (lookup_basic_face, realize_basic_faces)
(syms_of_xfaces):
* src/xdisp.c (window_box_height, window_text_pixel_size)
(display_mode_lines, Fformat_mode_line):
* src/dispextern.h (CURRENT_MODE_LINE_ACTIVE_FACE_ID_3)
(CURRENT_MODE_LINE_ACTIVE_FACE_ID, enum face_id): Rename from
*MODE_LINE_FACE_ID to *MODE_LINE_ACTIVE_FACE_ID.