Setup icon (i,j coordinates in ImGui Toolkit) for each Source class, and

use this icon in GUI to indicate the type of class.

DeviceSource.h

@@ -114,6 +114,8 @@ public:
     void setDevice(const std::string &devicename);
     inline std::string device() const { return device_; }
 
+    glm::ivec2 icon() const override;
+
 private:
     std::string device_;