fixup notification icon path code

1) Don't assume that game_config::path ends in '/'
2) Normalize the path after concatenating '/' + "images/foo.png",
to eliminate the double slashes "//" which apparently confuse dbus

src/desktop/dbus_notification.cpp

@@ -15,6 +15,7 @@
 #include "dbus_notification.hpp"
 #include "global.hpp"
 
+#include "filesystem.hpp"
 #include "game_config.hpp"
 #include "log.hpp"
 
@@ -145,8 +146,14 @@ uint32_t send_dbus_notification(DBusConnection *connection, uint32_t replaces_id
 			DBUS_TYPE_INVALID);
 	}
 
-	std::string app_icon_ = game_config::path + game_config::images::app_icon;
-	LOG_DU << "app_icon_=\'" << app_icon_ << "\'\n";
+	std::string app_icon_ = normalize_path(game_config::path + "/" + game_config::images::app_icon);
+	if (!file_exists(app_icon_)) {
+		ERR_DU << "Error: Could not find notification icon.\n"
+			<< "raw path =\'" << game_config::path << "\' / \'" << game_config::images::app_icon << "\'\n"
+			<< "normalized path =\'" << app_icon_ << "\'\n";
+	} else {
+		LOG_DU << "app_icon_=\'" << app_icon_ << "\'\n";
+	}
 
 	const char *app_icon = app_icon_.c_str();
 	const char *summary = owner.c_str();