Problem:
`nvim_win_set_config` cannot set the title and footer independently.
When only one is given, the other is reset to the default of "left".

Solution:
Reuse existing title/footer value if not provided.

Problem:  Combined highlighting was not applied to nvim_eval_statusline(),
          and 'statuscolumn' sign segment/numhl highlights.
Solution: Add an additional `groups` element to the return value of
          `nvim_eval_statusline()->highlights`. This is an array of stacked
          highlight groups (highest priority last). Also resolve combined
          highlights for the 'statuscolumn' sign segment/numhl highlights.
          Expose/synchronize some drawline.c logic that is now mimicked in
          three different places.

Problem:  Anchoring a floating window to the tabline and laststatus is
          cumbersome; requiring autocommands and looping over all
          windows/tabpages.
Solution: Add new "tabline" and "laststatus" options to the `relative`
          field of nvim_open_win() to place a window relative to.

Problem:
Since e049c6e4c08a, most statusline-like UI elements can combine
highlight attrs, except for sign/statuscolumn.

Solution:
Implement for sign/statuscolumn.

Problem: when zindex is changed in vim.schedule the zindex sort in
layers not changed.

Solution: resort layers when zindex changed.

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.

Problem:  Cannot allow mouse interaction for non-focusable float window.
Solution: Add a "mouse" field to float window config.