SVG support for menu.lua and utils.lua

I added support for the SVG image file format for menu.lua and utils.lua. SVG support is optional according to the freedesktop icon theme specification(http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html). Awful handles the SVG format fine, so I only changed some lines in the all_icon_sizes(also added 8x8) and the lookup function. In menu.lua requests were made specifically for PNG files, so I removed that too(I suppose it also enables XPM categories icons).