HIG cleanup

Clean up cinnamon-settings UI to be less messy and misaligned. Additionally it removes lots of copy&paste code, so the total LOC go down.

So far `cs_effects.py` has got the treatment. Additionally, I had to touch `SettingsWidget.py` packing options, since it forces `fill=False`, which is a very bad default for almost everything. Even if we don't expand, we almost certainly want to fill.

![before](https://f.cloud.github.com/assets/179487/845569/d943e206-f3ea-11e2-90fd-3aacf0ead8fb.png)
![after](https://f.cloud.github.com/assets/179487/845596/fbc69250-f3eb-11e2-90f0-c3fa9c7e90f3.png)