implement save as, save version and file existence check on save

engine/asset.cpp

@@ -18,6 +18,24 @@ bool Asset::delete_file(const std::string& path)
     return true;
 }
 
+bool Asset::exist(std::string path, bool is_asset)
+{
+    if (is_asset)
+    {
+        Asset asset;
+        if (asset.open(path.c_str()))
+        {
+            asset.close();
+            return true;
+        }
+    }
+    else
+    {
+        std::ifstream f(path);
+        return f.is_open();
+    }
+}
+
 std::vector<std::string> Asset::list_files(std::string folder, bool is_asset, const std::string& filter_regex)
 {
     std::vector<std::string> names;