Fix accessing to invalid Site object after undoing/redoing

Mainly the site.layer() field of the context writer after an undo/redo
is not valid anymore (e.g. because the layer could have been removed).
Now we get the new active site (Context::activeSite()) to go to that
place after the undo/redo operation.

src/app/commands/cmd_undo.cpp

@@ -130,9 +130,10 @@ void UndoCommand::onExecute(Context* context)
   // (because new frames/layers could be added, positions that we
   // weren't able to reach before the undo).
   if (gotoModified) {
+    Site newSite = context->activeSite();
     SpritePosition currentPosition(
-      writer.site()->layer(),
-      writer.site()->frame());
+      newSite.layer(),
+      newSite.frame());
 
     if (spritePosition != currentPosition) {
       Layer* selectLayer = spritePosition.layer();