perf: color palette on sheet switch ...

[mmeeks]

Reading the protocol logs it seems that before we get new tiles when switching sheet in a spreadsheet we get quite a big chunk of JSON built and sent to the client - which (I expect) helps to make the switching experience less slick than it could be.

We should also probably avoid fiddling with the UI until we have the new grid layout and tiles in-hand for the new sheet =)

Thanks ! =)