Don't set the palette if we change the current frame of an inactive Editor

This happens when we press the play button in one Editor (it changes to
PlayState) and we start using other Editor. We don't want to change
the current palette/ColorBar for that animation playing in the background.

Fix #676

src/app/ui/document_view.cpp

@@ -79,6 +79,7 @@ public:
   void onAfterFrameChanged(Editor* editor) override {
     updatePreviewEditor(this);
 
+    if (isActive())
       set_current_palette(editor->sprite()->palette(editor->frame()), true);
   }