Delete multiple selected event handlers from the sidebar

Deleting multiple selected event handlers from the member sidebar does not remove the whole selection. Choosing Delete Handler from the context menu removes only the handler that was right-clicked. Pressing Delete with several handlers selected removes none.

Both deletion paths should remove every selected handler together while keeping the controls and unselected handlers intact. The deletion should be one undoable action: Undo restores all deleted handlers and their code, and Redo removes the same group again.