Re-organize Nix Modules

- Move all programs into system folder
- Create `default.nix` for each folder to reduce number of imports
  in configuration.nix
- Rename some modules to be more generic and less package
  specific to allow for easy swapping of programs in the future
- Update flake.lock

system/gui/fonts.nix

@@ -2,6 +2,7 @@
 
 {
   fonts.packages = with pkgs; [
+    corefonts
     powerline
     font-awesome
     ubuntu_font_family