Fixed UI bug with buttons

On iOS users expect the button to respond to being pressed. With the tweak SwitchIcons when pressing the icons they darken slightly. When pressing other buttons on iOS they either glow or darken under your finger to indicate that they are being pressed.
Admittedly some might want to disable that behaviour. However, the behaviour should always default to the most likely scenario. And in most situations `adjustsImageWhenHighlighted = YES` is the correct decision.

When I first used the button API I actually thought the buttons were broken or I had somehow disabled them, that is how pervasive this little touch in iOS is and it should not be broken unless there is a good reason for it.