fontchooser: Set a fixed size on the preview cell

fontchooser: Set a fixed size on the preview cell

This way, all rows have the same height. It has 3 advantages:
1) No actual computation is necessary to compute the size of the cell.
   This speeds up the list tremendously (filling out the list goes from
   25s to 3s).
2) Buggy fonts don't mess up the list anymore with their weird sizes.
   Instead, they are clipped / empty space is added.
3) Buggy fonts look more buggy. So their use is hopefully discouraged.