Allow 8x8 and 12x12 emblems

As mentioned in #14, some projects ship 8x8 emblems. This MR adds 8x8 and 8x8@2 emblem directories to index.theme so that such icons are actually used - otherwise larger icons are downscaled and used instead.

Edit: Further to this, add 12x12 and 12x12@2 emblem directories to index.theme. I noticed that at least Nemo and Caja look for 12x12 emblems, and in my project I have emblems that are simplified at small sizes (<= 16x16) and then have more detail at larger sizes. From what I can tell so far, in order to do this I need to add icons to /usr/share/icons/hicolor in all the sizes <= 16x16 that an application might look for, otherwise they will use the scalable icon, which in my case is designed for larger sizes.