Handle undo when changing new item before placing.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21391

(cherry picked from commit a191d4e6e5)

include/commit.h

@@ -141,6 +141,8 @@ public:
                            UNDO_REDO aModFlag = UNDO_REDO::UNSPECIFIED,
                            BASE_SCREEN *aScreen = nullptr );
 
+    void Unstage( EDA_ITEM* aItem, BASE_SCREEN* aScreen );
+
     /// Execute the changes.
     virtual void Push( const wxString& aMessage = wxT( "A commit" ), int aFlags = 0 ) = 0;