Return number of lines available for display on FRAME.
If FRAME is omitted, describe the currently selected frame.
Exactly what is included in the return value depends on the
window-system and toolkit in use - see `frame-pixel-height' for
more details. The lines are in units of the default font height.
The result is roughly related to the frame pixel height via
height in pixels = height in lines * `frame-char-height'.
However, this is only approximate, and is complicated e.g. by the
fact that individual window lines and menu bar lines can have differing font heights.