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/Subsurface.cpp

@@ -236,7 +236,7 @@ Vector2D CSubsurface::coordsGlobal() {
     Vector2D coords = coordsRelativeToParent();
 
     if (m_pWindowParent)
-        coords += m_pWindowParent->m_vRealPosition.vec();
+        coords += m_pWindowParent->m_vRealPosition.value();
     else if (m_pPopupParent)
         coords += m_pPopupParent->coordsGlobal();