animations: Refactor AnimatedVariable (#4911)

* animation: Refactor AnimatedVariable

This commit decomposes the AnimatedVariable class into a base class
with the common attribute to all variable types and a templated derived
type containing strongly typed info on the type being animated.

Access to the typed version is perfomed using the visitor pattern. A
utility is provided to build a visitor on the fly using lambdas.

Adding a new type to be animated should just be a matter of adding the
typed in the list defined by the ANIMABLE_TYPES macro

The size of the commit is justified by the API change in the
AnimatedVariable class. No more vec(), fl() or col() method but a unified
value() method.

* animation: Remove visitor pattern

* animation: Fix coding style

* animation: Fix coding style

src/desktop/Popup.cpp

@@ -207,9 +207,9 @@ Vector2D CPopup::localToGlobal(const Vector2D& rel) {
 
 Vector2D CPopup::t1ParentCoords() {
     if (m_pWindowOwner)
-        return m_pWindowOwner->m_vRealPosition.vec();
+        return m_pWindowOwner->m_vRealPosition.value();
     if (m_pLayerOwner)
-        return m_pLayerOwner->realPosition.vec();
+        return m_pLayerOwner->realPosition.value();
 
     ASSERT(false);
     return {};