Add rasterization script using ImageMagick

The script makes png versions of the svg vector icons and places them in a
directory structure appropriate for GTK+ icon themes.
Merge branch 'feature/rasterize' into feature/custom_icons
Make menu icon borders more pronounced
Install and version rasterized icons for 16x16

Maintaining the rasterized icons as separate allows for fine-tuning after the
rasterization stage.