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;
+}

tests/modules/programs/feh/feh-bindings-expected-buttons

@@ -0,0 +1,4 @@
+zoom_in
+next_img C-4
+prev_img 3 C-3
+zoom_out 4

tests/modules/programs/feh/feh-bindings-expected-keys

@@ -0,0 +1,3 @@
+zoom_in
+prev_img h Left
+zoom_out minus

tests/modules/programs/feh/feh-bindings.nix

@@ -0,0 +1,33 @@
+{ pkgs, ... }:
+
+{
+  config = {
+    programs.feh.enable = true;
+
+    programs.feh.buttons = {
+      zoom_in = null;
+      zoom_out = 4;
+      next_img = "C-4";
+      prev_img = [ 3 "C-3" ];
+    };
+
+    programs.feh.keybindings = {
+      zoom_in = null;
+      zoom_out = "minus";
+      prev_img = [ "h" "Left" ];
+    };
+
+    nixpkgs.overlays =
+      [ (self: super: { feh = pkgs.writeScriptBin "dummy-feh" ""; }) ];
+
+    nmt.script = ''
+      assertFileContent \
+        home-files/.config/feh/buttons \
+        ${./feh-bindings-expected-buttons}
+
+      assertFileContent \
+        home-files/.config/feh/keys \
+        ${./feh-bindings-expected-keys}
+    '';
+  };
+}

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
+    '';
+  };
+}