Implement axis targets, axis -> button and axis -> relative axis mappings. (#1)

Co-authored-by: Anna Rose Wiggins <annabunches@gmail.com>
Co-committed-by: Anna Rose Wiggins <annabunches@gmail.com>

internal/mappingrules/rule_target_relaxis.go

@@ -0,0 +1,46 @@
+package mappingrules
+
+import (
+	"github.com/holoplot/go-evdev"
+)
+
+type RuleTargetRelaxis struct {
+	DeviceName string
+	Device     RuleTargetDevice
+	Axis       evdev.EvCode
+	Inverted   bool
+}
+
+func NewRuleTargetRelaxis(device_name string,
+	device RuleTargetDevice,
+	axis evdev.EvCode,
+	inverted bool) (*RuleTargetRelaxis, error) {
+
+	return &RuleTargetRelaxis{
+		DeviceName: device_name,
+		Device:     device,
+		Axis:       axis,
+		Inverted:   inverted,
+	}, nil
+}
+
+// NormalizeValue takes a raw input value and converts it to a value suitable for output.
+//
+// Relative axes are currently only supported for output.
+// TODO: make this have an error return?
+func (target *RuleTargetRelaxis) NormalizeValue(value int32) int32 {
+	return 0
+}
+
+func (target *RuleTargetRelaxis) CreateEvent(value int32, mode *string) *evdev.InputEvent {
+	return &evdev.InputEvent{
+		Type:  evdev.EV_REL,
+		Code:  target.Axis,
+		Value: value,
+	}
+}
+
+// Relative axis is only supported for output.
+func (target *RuleTargetRelaxis) MatchEvent(device RuleTargetDevice, event *evdev.InputEvent) bool {
+	return false
+}