Make the frame FRAME invisible.
If omitted, FRAME defaults to the currently selected frame.
On graphical displays, invisible frames are not updated and are
usually not displayed at all, even in a window system's "taskbar".
Normally you may not make FRAME invisible if all other frames are invisible,
but if the second optional argument FORCE is non-nil, you may do so.
This function has no effect on text terminal frames. Such frames are
always considered visible, whether or not they are currently being
displayed in the terminal. (fn &optional FRAME FORCE)