Synthesis of a Solid Solution Series of Layered EuxGd1−x(OH)2.5Cl0.5·0.9H2O and Its Transformation into (EuxGd1−x)2O3 with Enhanced Photoluminescence Properties (Synthesis of a Solid Solution Series of Layered EuxGd1−x(OH)2.5Cl0.5·0.9H2O and Its Transformation into (EuxGd1−x)2O3with Enhanced Photoluminescence Properties)