implement automatic color generation for new layer

GUI/Views/SampleDesigner/SampleEditorController.cpp

@@ -55,8 +55,43 @@ void SampleEditorController::addLayer(LayerItem* before)
     const int rowInMultiLayer = (before != nullptr) ? m_multiLayerItem->layers().indexOf(before)
                                                     : m_multiLayerItem->layers().size();
 
+    // -- find a color for the new layer
+    QColor color;
+    auto unusedColors = LayerEditorUtils::predefinedLayerColors();
+    for (auto l : m_multiLayerItem->layers())
+        unusedColors.removeAll(l->color());
+    if (!unusedColors.isEmpty())
+        color = unusedColors.first();
+    else {
+        // search for a color which has been used the less, and which is not the same as in the
+        // layers above and below
+        QMap<QString, int> usage;
+        for (auto l : m_multiLayerItem->layers())
+            usage[l->color().name()] += 1;
+
+        auto sortedByUsage = LayerEditorUtils::predefinedLayerColors();
+        std::stable_sort(
+            sortedByUsage.begin(), sortedByUsage.end(),
+            [&](const QColor& a, const QColor& b) { return usage[a.name()] < usage[b.name()]; });
+
+
+        const QColor above = (rowInMultiLayer > 0)
+                                 ? m_multiLayerItem->layers()[rowInMultiLayer - 1]->color()
+                                 : QColor();
+        const QColor below = (rowInMultiLayer < m_multiLayerItem->layers().size())
+                                 ? m_multiLayerItem->layers()[rowInMultiLayer]->color()
+                                 : QColor();
+
+        for (auto col : sortedByUsage)
+            if (col != above && col != below) {
+                color = col;
+                break;
+            }
+    }
+
+    // - create new layer
     LayerItem* layer = m_multiLayerItem->addLayer(rowInMultiLayer);
-    // #baLayerEditor set color of layer
+    layer->setColor(color);
 
     ASSERT(m_multiLayerForm);
     m_multiLayerForm->onLayerAdded(layer);