[shared/continous_function] Detach cache of inactive function

When a function that had previously been cached is deactivated, its
cache can be used by another function. When the function is reactivated,
if it tries to reuse its previous cache, it will be filled with values
from another function, and will not be cleared. By detaching the cache
of a function that becomes inactive, we ensure that the next time this
function is cached, the chache will be cleared.

e.g. Define to functions f(x)=x, g(x)=-x, draw them both.
     Deactivate f, the redraw the graph
     Reactivate f, the draw again

apps/shared/function.cpp

@@ -50,6 +50,9 @@ KDColor Function::color() const {
 
 void Function::setActive(bool active) {
   recordData()->setActive(active);
+  if (!active) {
+    functionBecameInactive();
+  }
 }
 
 int Function::printValue(double cursorT, double cursorX, double cursorY, char * buffer, int bufferSize, int precision, Poincare::Context * context) {