feat(keybinds): add options to specify keybinds

this adds two new options:
nixos95.keybinds.commands -> specify normal shortcuts
nixos95.keybinds.xfwm4    -> specify xfwm4 related shortcuts

nixos95/util/lib.nix

@@ -0,0 +1,6 @@
+{ lib, ... }: {
+  # return true if elm.enable is not set or elm.enable is true
+  isEnable = elm : !( lib.hasAttr "enable" elm ) || elm.enable;
+  # return executable path from elm.pkg if defined or elm.exe
+  getExe = elm : if elm ? pkg then lib.getExe elm.pkg else elm.exe;
+}