Preserving layout of zoomed windows across restores

The problem is that tmux list-window shows only the current pane layout
if a pane is maximized. This is a bug in tmux. In order to avoid this
bug we unzoom the window when saving and zoom in again after saving.
This implies that the Z flag is no longer set in list-windows, and so it
can't be used when restoring. Instead we use the Z flag of the panes
(which still have it) to restore the zoom.
Removing the last_resized variable

Instead resizing only if pane_active is set so we are sure the resizep
is called only once
Update changelog & readme for the zoomed windows feature

Closes #47, #54
v1.4.0