add iframe 'src' scripting property

src/graphics/ui/elements/InlineFrame.cpp

@@ -49,3 +49,7 @@ void InlineFrame::setSize(glm::vec2 size) {
         root->setSize(size);
     }
 }
+
+const std::string& InlineFrame::getSrc() const {
+    return src;
+}

src/graphics/ui/elements/InlineFrame.hpp

@@ -15,6 +15,8 @@ namespace gui {
 
         void act(float delta) override;
         void setSize(glm::vec2 size) override;
+
+        const std::string& getSrc() const;
     private:
         std::string src;
         std::shared_ptr<UiDocument> document;