Center first window on every workspace

Some participants in our research had issues with odd default window positions and sizes. It also just looks very weird when windows open off-center, especially for the first window. Longer-term I'd be interested in exploring something like tiling-first, but for now centering the first window on every workspace (excluding special cases like half-tiled windows) seems like a nice step forward without requiring any major changes.

cc @mcatanzaro @aday