[plasmoidconfig] Fix sizing for some configurations
We'd like to rely on the config to give us an appropriate implicitHeight, but some configs do the opposite and assume they fill the parent. The old code resolved this by making the config at least as high as its parent. Reintroduce that.
This is arguably all very ugly and should be resolved properly, but that requires changing the configs themselves, which is infeasible for Plasma 5 with regards to third-party applets