String plug widget fixes

This fixes bugs in the StringPlugValueWidget and MultiLineStringPlugValueWidget, which became apparent when using Backdrops. I did mail Paulo privately about the changes in the first commit but didn't hear back - but I have looked at the Jabuka code and as far as I can tell, the change is fine. If not, let me know and we can back out that one commit for now - but I do intend to tidy this up before long either way.