feh: allow binding actions to multiple buttons/keys

In feh you can bind multiple keys to the same action, but Home Manager
only let you set a single key to an action. You can cheat and pass a
string with space-separated keys, but with this change you can pass a
list for each action to bind multiple keys to it.

Also adds a couple of tests.

Fixes #1366

tests/modules/programs/feh/feh-empty-settings.nix

@@ -0,0 +1,15 @@
+{ pkgs, ... }:
+
+{
+  config = {
+    programs.feh.enable = true;
+
+    nixpkgs.overlays =
+      [ (self: super: { feh = pkgs.writeScriptBin "dummy-feh" ""; }) ];
+
+    nmt.script = ''
+      assertPathNotExists home-files/.config/feh/buttons
+      assertPathNotExists home-files/.config/feh/keys
+    '';
+  };
+}