terminal: Open a new window instead of split on older tmux versions (<1.9)

terminal: Open a new window instead of split on older tmux versions (<1.9)

If an old version is detected (<1.9), create a new window instead of split:
the reason is that there is no easy way to get the active pane height if no
nested formats are supported.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>