Use SVG images for checkboxes and radio buttons

[Christopher Kyle Horton]

Rather than deal with the mess of limited options GTK+3.6 itself would provide us to try and fix the checkboxes and radio buttons, we should instead adopt Ambiance's approach of using custom images for these UI elements. Currently, they are both broken and ugly, and this approach would not only fix that but also allow us greater control over how the design looks.

Resolving this bug involves three parts:
1. Create images to use for the checkboxes and radio buttons.
2. Modify the GTK+3 code to use these images for better appearance.
3. Modify the GTK+2 code to use these images for consistency.