implement work_path for documents and data_path for assets

src/app.h

@@ -66,6 +66,9 @@ public:
     NodeScroll* panels;
     const uint16_t main_id = const_hash("main");
     std::string doc_name = "no-name";
+    std::string doc_path;
+    std::string doc_dir;
+    std::string doc_filename;
     float width;
     float height;
     bool keys[256];