Implications for the Thin-Film Densification of TiO2 from Carboxylic Acid-Modified Titanium Alkoxides. Syntheses, Characterizations, X-ray Structures of Ti3(μ3-O)(O2CH)2(ONep)8, Ti3(μ3-O)(O2CMe)2(ONep)8, Ti6(μ3-O)6(O2CCHMe2)6(ONep)6, [Ti(μ-O2CCMe3)(ONep)3]2, and Ti3(μ3-O)(O2CCH2CMe3)2(ONep)8 (ONep = OCH2CMe3)1