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>

app/internal/window/os_wayland.go

@@ -801,11 +801,11 @@ func gio_onPointerButton(data unsafe.Pointer, p *C.struct_wl_pointer, serial, t,
 	var btn pointer.Buttons
 	switch wbtn {
 	case BTN_LEFT:
-		btn = pointer.ButtonLeft
+		btn = pointer.ButtonPrimary
 	case BTN_RIGHT:
-		btn = pointer.ButtonRight
+		btn = pointer.ButtonSecondary
 	case BTN_MIDDLE:
-		btn = pointer.ButtonMiddle
+		btn = pointer.ButtonTertiary
 	default:
 		return
 	}