Fading of propertyowners through GUI, and more owners with fade property (#2557)

* Add property in OpenSpaceEngine for fading duration when enabling / disabling

* Add interface for fadeable components and apply to renderable

* Make layers a fadeable component

* Make globelabels a fadeable component

* Make labelscomponent a fadeable, and give it an enabled property

* Make screenspace renderables fadeable components

* Introduce concept of parent fadeables, to fade out subowners with parent

* Make rings fadeable as well

---------

Co-authored-by: Alexander Bock <alexander.bock@liu.se>

include/openspace/rendering/renderable.h

@@ -26,6 +26,7 @@
 #define __OPENSPACE_CORE___RENDERABLE___H__
 
 #include <openspace/properties/propertyowner.h>
+#include <openspace/rendering/fadeable.h>
 
 #include <openspace/properties/scalar/boolproperty.h>
 #include <openspace/properties/scalar/doubleproperty.h>
@@ -52,7 +53,7 @@ namespace documentation { struct Documentation; }
 
 class Camera;
 
-class Renderable : public properties::PropertyOwner {
+class Renderable : public properties::PropertyOwner, public Fadeable {
 public:
     enum class RenderBin : int {
         Background = 1,
@@ -102,9 +103,7 @@ public:
 
     bool matchesSecondaryRenderBin(int binMask) const noexcept;
 
-    void setFade(float fade);
-
-    bool isVisible() const;
+    bool isVisible() const override;
 
     void onEnabledChange(std::function<void(bool)> callback);
 
@@ -112,8 +111,6 @@ public:
 
 protected:
     properties::BoolProperty _enabled;
-    properties::FloatProperty _opacity;
-    properties::FloatProperty _fade;
     properties::StringProperty _renderableType;
     properties::BoolProperty _dimInAtmosphere;
 
@@ -124,7 +121,7 @@ protected:
     void registerUpdateRenderBinFromOpacity();
 
     /// Returns the full opacity constructed from the _opacity and _fade property values
-    float opacity() const;
+    float opacity() const override;
 
     double _boundingSphere = 0.0;
     double _interactionSphere = 0.0;