Refactor statusline elements to build Spans

[hamrik]

The current statusline implementation uses ad-hoc write callbacks to render statusline elements. This make statusline elements hard to work with, because we get no useful information about their output.

This PR refactors these elements to work more like widgets: each render_* function returns a Spans, which can be combined into a complete statusline. This has remained the responsibility of the render function.

This PR also separates the render function into two parts: render_statusline combines the aforementioned elements into one large Spans with given size constraints, while render blits it to a given Surface as it has done so previously.

The rationale behind this is to allow post-processing the widget outputs and to make decisions based on them.
For example, in the future we can now check if the statusline is empty and autohide it. Or revisit #7310 and automatically space the elements.

This is merely a refactor; the current apparent behavior of each statusline element should be identical to previous versions.

[the-mikedavis]

This just needs a rebase / master merge to resolve the conflicts

[hamrik]

Rebase done!