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>

io/router/pointer_test.go

@@ -488,7 +488,7 @@ func TestCursorNameOp(t *testing.T) {
 		return pointer.Event{
 			Type:     pointer.Move,
 			Source:   pointer.Mouse,
-			Buttons:  pointer.ButtonLeft,
+			Buttons:  pointer.ButtonPrimary,
 			Position: f32.Pt(x, y),
 		}
 	}