st-button: Ignore pointer emulated touch events

st-button: Ignore pointer emulated touch events

In X11, pointer emulated touch events are replicated with normal PRESS, RELEASE
pair events which are generated by the server. Thus for a single tap we get:
 - TOUCH_BEGIN -> TOUCH_END, PRESS -> RELEASE

This will cause st-button to send two "clicked" signals, instead of just one,
breaking extensions (like dash-to-dock) that show buttons in the main stage
which will be checked two times or that will receive the same signal two times.