keys: added member field initializers for KeyInputData.¹

1: Duh. Per valgrindin'.
tty::Event: added member field initializers.
ui::Context: added member field initializer for m_fallback_group.
ui::Field: changed floating-point op to an integer op (x*0.5 to x/2).
geometry: added value-range value_clamp() variant.
test: moved iostream inspectors from ui/packing to common.