enable Edit menu and add Clear Presets feature

src/node_panel_brush.cpp

@@ -361,6 +361,11 @@ bool NodePanelBrush::restore()
     return false;
 }
 
+void NodePanelBrush::clear()
+{
+    m_container->remove_all_children();
+}
+
 void NodePanelBrush::added(Node* parent)
 {
     m_interacted = false;
@@ -604,6 +609,11 @@ void NodePanelBrushPreset::add_brush(std::shared_ptr<Brush> brush)
     b->on_click = std::bind(&NodePanelBrushPreset::handle_click, this, std::placeholders::_1);
 }
 
+void NodePanelBrushPreset::clear_brushes()
+{
+    m_container->remove_all_children();
+}
+
 void NodePanelBrushPreset::added(Node* parent)
 {
     m_interacted = false;