Implement Lua Flex widget

There needs to be a declarative way to scale widget sizes responsively.

I'm aiming for a simpler, one-dimensional (either row or column) take on a flexbox https://css-tricks.com/snippets/css/a-guide-to-flexbox/

Something like

local windowHeaderLayout = {
  type = ui.TYPE.Flex,
  props = {
    size = v2(300, 50),
    -- horizontal (row) by default, otherwise vertical (column)
    horizontal = true,
    -- align the widgets along the Flex's direction. Unused if any content widgets can grow
    justify = ui.ALIGNMENT.Center,
    -- align the widgets perpendicular the Flex's direction.
    align = ui.ALIGNMENT.Center,
  },
  content = ui.content {
    { -- Window header left block
      type = ui.TYPE.Image,
      -- ... properties of the image
      external = {
        grow = 1, -- can grow to take up free space
      },
    },
    { -- Window caption, autosized by default
      type = ui.TYPE.Text,
      props = {
        text = getCaption(), -- some dynamic caption of unknown size
      },
      external = {
        grow = 0, -- doesn't grow (default)
      },
    },
    { -- Window header right block
      type = ui.TYPE.Image,
      -- ... properties of the image
      external = {
        grow = 1, -- can grow to take up free space
      },
    },
  },
}