Automatic depth increment when placing a source.

View.h

@@ -89,7 +89,7 @@ public:
     void zoom (float factor) override;
     void grab (glm::vec2 from, glm::vec2 to, Source *s, std::pair<Node *, glm::vec2> pick) override;
 
-    void setDepth (Source *, float d);
+    void setDepth (Source *, float d = -1.f);
 
 private:
     float aspect_ratio;