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>

widget/example_test.go

@@ -44,13 +44,13 @@ func ExampleClickable_passthrough() {
 	r.Queue(
 		pointer.Event{
 			Source:   pointer.Mouse,
-			Buttons:  pointer.ButtonLeft,
+			Buttons:  pointer.ButtonPrimary,
 			Type:     pointer.Press,
 			Position: f32.Pt(50, 50),
 		},
 		pointer.Event{
 			Source:   pointer.Mouse,
-			Buttons:  pointer.ButtonLeft,
+			Buttons:  pointer.ButtonPrimary,
 			Type:     pointer.Release,
 			Position: f32.Pt(50, 50),
 		},