LibGUI: Fix undo stack reporting wrong modified state

Since the `redo` action never goes back to `index: 0`,
we have to mark the clean index as being the current
non-empty index for the undo/redo navigation to work properly.

The problem is that if we never `undo`, the stack index stays at zero,
which is the empty container waiting for commands. In that situation,
if we save the document, it registers the clean index as being 1
(the non-empty index) but because the stack index has never left zero,
the document was being reported as modified, being out of sync with
the window modified state.

Userland/Libraries/LibGUI/UndoStack.h

@@ -32,6 +32,8 @@ public:
     void clear();
 
 private:
+    size_t non_empty_stack_index() const;
+
     struct Combo {
         NonnullOwnPtrVector<Command> commands;
     };