Allow iterating all windows at the mux layer.
windows allow iterating tabs.
Tabs allow iterating panes.
Versions of iteration that report additional information (like index,
active and positioning) are also added.
Panes can now reference their containing tab and window objects.
Tabs can now reference their containing window object.
refs: https://github.com/wez/wezterm/issues/1598 (sort of)
refs: https://github.com/wez/wezterm/issues/225
