Rebranding, part deux

[voidanix]

• cmake: nix: Update icon packaging
• cmake: nix: Add 256 and 512 icon variants
• install: win: Remove remnants of NSIS installer
• install: win: Move Windows-specific files from src
• scripts: Add icon generation script
• install: nix: Update icons
• install: win: Use BN ICO
• data: Update textures submodule

[voidanix]

Hmm, seems to conflict with https://github.com/blue-nebula/base/pull/262: marking as WIP.

[TheAssassin]

Please feel free to ignore #262, we'll fix that one after the icons have been fixed.

[voidanix]

Ready for review now, made #262 happy by adding new bitmaps.

[TheAssassin]

Thanks!