split app.cpp into multiple files, add NodeScroll and use it to scroll the side panels, some fixes to the events system

engine/node_scroll.h

@@ -0,0 +1,14 @@
+#pragma once
+#include "node_border.h"
+
+class NodeScroll : public NodeBorder
+{
+    bool m_dragging = false;
+    glm::vec2 m_drag_start;
+    glm::vec2 m_offset_start;
+    glm::vec2 m_offset;
+    glm::vec2 m_mask{ 0, 1 };
+public:
+    virtual Node* clone_instantiate() const override;
+    virtual kEventResult handle_event(Event* e) override;
+};