terminal.py: add tmux new window option

Add a new terminal type that makes a new window in the running tmux
session instead of splitting the window. 80x25 is not enough to run
menuconfig inside a split window, so add the option to create a new
window instead.

Use the new window option by default when the split window would be
less than 19 lines high.

Signed-off-by: Dan McGregor <dan.mcgregor@usask.ca>
Signed-off-by: Ross Burton <ross.burton@intel.com>