Instead of tracking gaps per child apply gaps in two logical places:
1. In tiled containers use the layout code to add the gaps between
windows. This is much simpler and guarantees that the sizing of children
is correct.
2. In the workspace itself apply all the gaps around the edge. Here
we're in the correct position to size inner and outer gaps correctly and
decide on smart gaps in a single location.
Fixes#4296
d0233af3b3