fix: allow deviating paddings per side

[ttytm]

This PR allows the use of deviating paddings per side.

Currently, it won't work to set another padding for top then for bottom for example. It also cleans up variables a bit.

Here are some screenshots:

Adding padding top has no effect:

Changing padding bot changes both:

Allowing deviating values, e.g. top = 1, bot = 0 works quite nice with cmdheight = 0. Looks uniform.

Please let me know about desired changes and feel free to modify the PR.

[ttytm]

Hey @folke, hope you are good! When there is not too much work, let me know if this one can make it.

I'm cleaning up some forks and branches and seen it would be a quick review.

I updated the PR description to include some screenshots.

[folke]

Thanks!