Fourier-transform spectroscopy of (4) 1 Σ + → A 1 Σ + - b 3 Π, A 1 Σ + - b 3 Π → X 1 Σ + , and (1)^3∆ _1rArr b{ }^3Π _{0^± } transitions in KCs and deperturbation treatment of A 1 Σ + and b 3 Π states