Tools not interactive after layout switch

[PaulHax]

Reproduce:

1. Load prostate
2. Draw Ruler in upper left viewport
3. Switch Layout to Axial Only
4. Switch Layout to Quad View
5. Bug: Click to drag Ruler handle. But nothing happens

More info

After droping another tool in the same viewport, the broken tool becomes interactive again.

WidgetRepresentations are not getting rendered after layout switches.

[floryst]

I think I know what's happening here. I'm pretty sure I fixed this in one of my unmerged widget branches.