WIP: implement zindex option

[l-kershaw]

This implements the zindex option for the main window as done in vim, with hardcoded default, minimum and maximum values to match vim.

TODO:

• decide how best to handle zindex of the border (maybe 1 less than the zindex of the main? would then need to handle zindex=1 as a special case)
• implement zindex for borders in plenary
• pass decided zindex option when creating the border

[l-kershaw]

Closing as replacement PR in plenary has been merged