More intuitive padding specification

[aplavin]

Currently, padding for objects (eg Label) is specified as four numbers, padding=(a, b, c, d).
I personally find it very hard to remember the order, and basically never get it right on the first try. Probably the issue lies in the apparent similarity with CSS (it also uses four numbers), but a completely different order: (l, r, b, t) in Makie and (t, r, b, l) in CSS.

Can we use a more intuitive interface? Some potential examples:

• Two intervals instead of four numbers: padding=(0 ± 5, 0 ± 2) for 5px horizontal and 2px vertical, or (-3..5, -2..1) for different offsets in all four directions. This is already easier to understand, the l/r and t/b order is clear, only hor-vert remains, but it is more natural (x always comes before y).
• An explicit rectangle, like padding=Rect(0 ± 5, 0 ± 2). Slightly more verbose, but even more understandable and can neatly generalize to other shapes. Currently used in MakieExtra.textwithbox to try out this specification in practice.

I think either of these would be an improvement. Maybe even both should be supported, with a tuple-of-intervals being converted to a rect under the hood. Rects would play especially nicely if GeometryBasics could define a dilate(shape_a, shape_b) morphological operation.
Or maybe something different would be even better here?..

[ffreyer]

Makie.jl/src/conversions.jl

Lines 2036 to 2038 in 381cf4a

	to_lrbt_padding(x::Real) = Vec4f(x)
	to_lrbt_padding(xy::VecTypes{2}) = Vec4f(xy[1], xy[1], xy[2], xy[2])
	to_lrbt_padding(pad::VecTypes{4}) = to_ndim(Vec4f, pad, 0)

We could extend this to include a..b and Rect(l, b, w, h)