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/default.nix

@@ -0,0 +1,4 @@
+{
+  feh-empty-config = ./feh-empty-settings.nix;
+  feh-bindings = ./feh-bindings.nix;
+}