Some smaller square icons

[ubuntujaggers]

Hi everyone,

This is seeking to address #4163. I can either add the rest of the icons to this branch before we merge it, or we can just do multiple PRs to merge icons that are ready (assuming that people are happy with the new icons!).

The full size icon is still slightly bigger than I would like, because I would have liked the length of each side to be exactly halfway between the short and long sides of an oblong. Since the oblongs are 224 x 176, that would have meant a square of size 200 x 200. Unfortunately, when scaled to app grid size, 200px didn't shrink down to a whole number of pixels, meaning there was anti-aliasing round the edges. I've gone for 208 x 208px squares which are still noticeably smaller but render perfectly crisply on the app grid.

The revised icons in this branch (so far!) are as follows:

transmission
sudoku-app
tweaks-app
accessories-character-map
weather-app
wine
winetricks
session-properties
calculator-app
screenshot-app