grid: don't overwrite nested graph label positions

[gavin-ts]

Summary

Fixes grid layout "initializing" label and icon positions even if a nestedGraph layout already positioned them.

Details

• if a grid cell contains a nested graph, layout will run and position the cells label and/or icon, so we shouldn't overwrite that
• adds grid_icons test
• In the test below, the container label positions were incorrectly being overwritten
• This also adds spacing for these labels when it would conflict with the grid's label
• Icons are now also considered for this spacing
• factors out some d2graph methods into d2graph/layout.go

before/after

grid_icons before/after

[alixander]

if a grid cell contains a nested graph, layout will run and position the cells label and/or icon,

can the test include an icon example?

[gavin-ts]

can the test include an icon example?

added a test for icons and fixed the layouts to account for icons