[core] Return {Source,Layer}::getID by value

Avoid dangling references in the following sequence:

    auto& id = layer->getID();
    layer->setMaxZoom(2);
    std::cout << id;

The reference would be dangling because mutating the layer allocates a new Immutable impl, and there may be no references to the prior impl, which held the id.

include/mbgl/style/layer.hpp

@@ -92,7 +92,7 @@ public:
     }
 
     LayerType getType() const;
-    const std::string& getID() const;
+    std::string getID() const;
 
     // Visibility
     VisibilityType getVisibility() const;

include/mbgl/style/source.hpp

@@ -56,7 +56,7 @@ public:
     }
 
     SourceType getType() const;
-    const std::string& getID() const;
+    std::string getID() const;
     optional<std::string> getAttribution() const;
 
     // Private implementation

src/mbgl/style/layer.cpp

@@ -18,7 +18,7 @@ LayerType Layer::getType() const {
     return baseImpl->type;
 }
 
-const std::string& Layer::getID() const {
+std::string Layer::getID() const {
     return baseImpl->id;
 }
 

src/mbgl/style/source.cpp

@@ -19,7 +19,7 @@ SourceType Source::getType() const {
     return baseImpl->type;
 }
 
-const std::string& Source::getID() const {
+std::string Source::getID() const {
     return baseImpl->id;
 }