improve progress bar, refactor node destruction, simplify layers export

src/node_progress_bar.cpp

@@ -20,5 +20,17 @@ void NodeProgressBar::init()
     btn_cancel->on_click = [&](Node*) { destroy(); };
     m_progress = find<NodeBorder>("progress");
     
-    m_progress->SetWidthP(10);
+    m_progress->SetWidthP(0);
+}
+
+void NodeProgressBar::increment() noexcept
+{
+    m_count++;
+    if (m_total != 0)
+        m_progress->SetWidthP(((float)m_count / m_total) * 100.f);
+}
+
+void NodeProgressBar::set_progress(float p) noexcept
+{
+    m_progress->SetWidthP(p * 100.f);
 }