grid: support custom label/icon positions for grid containers

[gavin-ts]

currently grid containers always have labels inside top center, but they should be able to be positioned with label.near

For example

grid: {
  label.near: outside-top-left
  grid-rows: 3
  a
  b
  c
}

[alixander]

note that if labels are set to outside, then there should be no box for them as part of the grid.