Introduce the style::Layer factory classes

This patch introduces the initial implementation of
- A `LayerFactory` abstract class that creates `style::Layer` instances of
  a certain layer type (line, hillshade, round, ..)
- A singleton `LayerManager` class, which is responsible for initializing
  the `LayerFactory` instances and forwarding the `create()` calls to the
  corresponding factory.

src/mbgl/style/layers/circle_layer.cpp

@@ -687,5 +687,24 @@ Mutable<Layer::Impl> CircleLayer::mutableBaseImpl() const {
     return staticMutableCast<Layer::Impl>(mutableImpl());
 }
 
+CircleLayerFactory::~CircleLayerFactory() = default;
+
+const char* CircleLayerFactory::type() const {
+    return "circle";
+}
+
+std::unique_ptr<style::Layer> CircleLayerFactory::createLayer(const std::string& id, const conversion::Convertible& value) {
+    optional<std::string> source = getSource(value);
+    if (!source) {
+        return nullptr;
+    }
+
+    std::unique_ptr<style::Layer> layer = std::unique_ptr<style::Layer>(new CircleLayer(id, *source));
+    if (!initSourceLayerAndFilter(layer.get(), value)) {
+        return nullptr;
+    }
+    return layer;
+}
+
 } // namespace style
 } // namespace mbgl