Update debian packaging to work with Launchpad PPA

I dusted off Callum's old Debian dpkg scripts to make them work with Launchpad. Check it out -- now Ubuntu users can install SRB2 easily!

Both i386 and amd64 builds are provided by Launchpad from the source packages.

Significant changes:

• Assets: Only download asset files from srb2.org if they do not yet exist in the build folder
• Assets: Split the clean routine so that clean only touches assets/debian/tmp, while clean-all also cleans the asset files from srb2.org
• Assets: Package the asset files into the source .tar.xz, because Launchpad can't resolve srb2.org in their buildbots
• Main: Comment out the pkg-config hacks so that i386 Launchpad builds work correctly
• Main: Move the binary procedure to binary-arch because Launchpad only calls the latter
• Main: Manually add the GME linker flag (-lgme) if pkg-config cannot resolve GME
• Fix the .desktop icon so that it displays correctly in Ubuntu's program manager. Also update the PNG icon.