Improve down/up event handling of axes

[oznogon]

Axis bindings, including some buttons that report as axes, can emit negative values that Keybinding::setValue() handles inconsistently with regard to setting down_event and up_event. This can cause button presses or axis movements that should trigger events to fail only because they emit negative values (left/down stick movements, certain triggers or sliders that rest at 0 and report negative values in one direction).

Movement axes arguably shouldn't trigger down_/up_events at all, so ignore mouse and joystick movement axes, but not game controller axes, which are often pressure-sensitive buttons or triggers. For other axes, handle negative values when calculating whether they exceed the threshold to trigger the event.