Add seperate cache path

This is set to the XDG cache path where possible.
It's set to the app's cache path on Android.

src/porting.h

@@ -147,12 +147,23 @@ extern std::string path_user;
 */
 extern std::string path_locale;
 
+/*
+	Path to directory for storing caches.
+*/
+extern std::string path_cache;
+
 /*
 	Get full path of stuff in data directory.
 	Example: "stone.png" -> "../data/stone.png"
 */
 std::string getDataPath(const char *subpath);
 
+/*
+	Move cache folder from path_user to the
+	system cache location if possible.
+*/
+void migrateCachePath();
+
 /*
 	Initialize path_*.
 */