Improved handling of race icons (fixes #2030)

* The editor_icon= key will now be respected in the Unit Preview Pane
* If no appropriate icon is found, a generic icon will be used instead, in both the editor
  and unit preview pane.

src/editor/palette/unit_palette.cpp

@@ -50,11 +50,7 @@ void unit_palette::setup(const config& /*cfg*/)
 		config cfg;
 		cfg["id"] = i.second.id();
 		cfg["name"] = i.second.plural_name();
-		if(i.second.editor_icon().empty()) {
-			cfg["icon"] = "icons/unit-groups/race_" + i.second.id();
-		} else {
-			cfg["icon"] = i.second.editor_icon();
-		}
+		cfg["icon"] = i.second.get_icon_path_stem();
 		cfg["core"] = true;
 		groups_.emplace_back(cfg);
 	}

src/gui/widgets/unit_preview_pane.cpp

@@ -284,7 +284,7 @@ void unit_preview_pane::set_displayed_type(const unit_type& type)
 	}
 
 	if(icon_race_) {
-		icon_race_->set_label("icons/unit-groups/race_" + type.race_id() + "_30.png");
+		icon_race_->set_label(type.race()->get_icon_path_stem() + "_30.png");
 	}
 
 	if(icon_alignment_) {

src/units/race.cpp

@@ -19,6 +19,7 @@
 
 #include "units/race.hpp"
 
+#include "filesystem.hpp"
 #include "log.hpp"
 #include "serialization/string_utils.hpp"
 #include "serialization/unicode_cast.hpp"
@@ -145,3 +146,20 @@ unit_race::GENDER string_gender(const std::string& str, unit_race::GENDER def) {
 	}
 	return def;
 }
+
+std::string unit_race::get_icon_path_stem() const
+{
+	if(!icon_.empty()) {
+		return icon_;
+	}
+
+	std::string path = "icons/unit-groups/race_" + id_;
+
+	// FIXME: hardcoded '30' is bad...
+	if(!filesystem::file_exists(filesystem::get_binary_file_location("images", path + "_30.png"))) {
+		std::cerr << "path not found: " << "images/" + path + "_30.png" << std::endl;
+		path = "icons/unit-groups/race_custom";
+	}
+
+	return path;
+}

src/units/race.hpp

@@ -45,6 +45,21 @@ public:
 	unsigned int num_traits() const;
 	const std::string& undead_variation() const { return undead_variation_; }
 
+	/**
+	 * Gets this race's icon path without state/size suffix and extension.
+	 *
+	 * This doesn't return the full path. Its output looks something like this:
+	 * @c icons/unit-groups/race_elf
+	 *
+	 * This is because this output is used in the editor for a GUI1 button's icon,
+	 * and GUI1 automatically appends the state extension, such as "_30-pressed.png"
+	 *
+	 * If a custom icon has been provided, that is used. Else, it checks if an
+	 * appropriate icon exists in icons/unit-groups/. If not, a generic custom race
+	 * icon will be used.
+	 */
+	std::string get_icon_path_stem() const;
+
 	/// Dummy race used when a race is not yet known.
 	static const unit_race null_race;