Novel Phosphorus Cations. 3. Derivatives of the Phosphaalkyne Tetramer 1,2,5,6-Tetraphosphatricyclo[4.2.0.02,5]octadiene: Phosphonium Ions of Alkylation (EtOTf) and Acylation (MeCO+ SbCl6-), and Mono- and Diprotonation with Superacids; Synthesis of the 1-Monooxo, 1-Monothioxo, 1-Tosylimino, and 1,5-Ditosylimino Derivatives