Define a grid system

We should use a predefined grid for laying out nearly every part of our OS. We already have things like 12px margins _between_ some items, but it could be beneficial to line as much as we can up to a coherent grid.

Things to look out for:
- a grid is a guideline, not a hard rule
- dealing with translatable strings means things can move around
- take baby steps, i.e. make sure Granite widgets follow a grid first, then typography, etc.