Adding playfield items takes too long

[arthurkehrwald]

Not a big deal, but the editor freezes up for up 0.5 to 3 seconds when adding a new item using the toolbox window. The delay seems to be higher the first time.

[freezy]

Yes, it's weird, I've been debugging this already once and haven't come to any conclusion. Will have another go, otherwise I'll change the approach on how to add those items (right now they are instantiated like an import, which does lots of things that can be skipped).

[freezy]

See also #539