New streamlined icons for the GUI. Add a back to work icon.

Fixes #2296
Fixes #4019
Thread:
https://wildfiregames.com/forum/index.php?/topic/27738-experimenting-with-icons/&tab=comments#comment-393054

This was SVN commit r23631.

binaries/data/mods/public/gui/session/unit_actions.js

@@ -1101,8 +1101,6 @@ var g_UnitActions =
 	},
 };
 
-var g_UnitActionsSortedKeys = Object.keys(g_UnitActions).sort((a, b) => g_UnitActions[a].specificness - g_UnitActions[b].specificness);
-
 /**
  * Info and actions for the entity commands
  * Currently displayed in the bottom of the central panel
@@ -1304,7 +1302,7 @@ var g_EntityCommands =
 			return {
 				"tooltip": colorizeHotkey("%(hotkey)s" + " ", "session.backtowork") +
 				           translate("Back to Work"),
-				"icon": "production.png"
+				"icon": "back-to-work.png"
 			};
 		},
 		"execute": function()