Suggestion to make the metatile layers more clear

[Rangi42]

Currently the Metatile Properties dialog shows two layers side by side labeled "Bottom/Top". This is fairly arbitrary (why not show Top on the left?) and doesn't change based on the chosen Layer Type (e.g. Middle/Top).

Proposal: show all three layers vertically stacked, with graphics in whichever two are being applied by the current Layer Type (or all three with triple-layer tiles).