Fix icons (default kivy icons for 24px, 256px, and 512px)

[maltfield]

Describe the bug
Currently not all of the default kivy icons are being replaced by the BusKill icon

When I was working on issue #16 & #39 today, I realized that the KIVY_HOME dir has an icons dir, and inside that dir has a mix of kivy's default icons and our custom BusKill icons. I guess that I didn't add all of the sizes needed, so there's three files that are still the default kivy icon:

1. kivy-icon-24.png
2. kivy-icon-256.png
3. kivy-icon-512.png

See screenshot of my dev machine's icons dir inside KIVY_HOME (/home/user/.local/share/.buskill/icon/):