Swap physical buttons

[usr-sse2]

On touchpads with one integrated button, left-handed users will be able (after fix of #496) to click the bottom-left corner to make a right click if it is configured in System Preferences.
However, touchpads with two physical buttons will not be affected by this setting.
A possible partial solution would be to read this setting and swap buttons in emitted events, but in fact the only useful option on such touchpads is two-finger tap/click, which is symmetric, so it should be configured in some way outside of Trackpad prefpane.
It's also unclear why mouse preferences do not affect this.