Minor glitches when switching levels while creating edges

[mxgrey]

#411 went a long way towards fixing the edge case of switching levels while in the middle of creating edges, but I've noticed there is are still some minor glitches:

• Sometimes after switching levels, a large wall (or other edge-based object) will glitch into the scene for one frame and disappear
• If you switch levels before selecting the start point of the edge, the first click on the new level will yield an empty anchor that goes unused.

Since these glitches don't particularly break anything, we can save them for after the first release.