io/pointer: rename button names to reflect their meaning, not placement

For example, ButtonLeft may be the right-most button for a left-handed user.
Rename the button names to match their intended use.

This is an API change. Use the following commands to update your
projects:

    $ gofmt -r 'pointer.ButtonLeft -> pointer.ButtonPrimary' -w .
    $ gofmt -r 'pointer.ButtonRight -> pointer.ButtonSecondary' -w .
    $ gofmt -r 'pointer.ButtonMiddle -> pointer.ButtonTertiary' -w .

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gesture/gesture.go

@@ -172,7 +172,7 @@ func (c *Click) Events(q event.Queue) []ClickEvent {
 			if c.pressed {
 				break
 			}
-			if e.Source == pointer.Mouse && e.Buttons != pointer.ButtonLeft {
+			if e.Source == pointer.Mouse && e.Buttons != pointer.ButtonPrimary {
 				break
 			}
 			if !c.entered {
@@ -341,7 +341,7 @@ func (d *Drag) Events(cfg unit.Metric, q event.Queue, axis Axis) []pointer.Event
 
 		switch e.Type {
 		case pointer.Press:
-			if !(e.Buttons == pointer.ButtonLeft || e.Source == pointer.Touch) {
+			if !(e.Buttons == pointer.ButtonPrimary || e.Source == pointer.Touch) {
 				continue
 			}
 			if d.dragging {

gesture/gesture_test.go

@@ -64,7 +64,7 @@ func mouseClickEvents(times ...time.Duration) []event.Event {
 	press := pointer.Event{
 		Type:    pointer.Press,
 		Source:  pointer.Mouse,
-		Buttons: pointer.ButtonLeft,
+		Buttons: pointer.ButtonPrimary,
 	}
 	events := make([]event.Event, 0, 2*len(times))
 	for _, t := range times {