Add setTextValidityExpire

This function change color from green to gray of text in canvas attached to
vscpws_simpleTextEvent, if no new message received within <interval> ms.

Usage:
var txt1 = new vscpws_simpleTextEvent(...);
txt1.setTextValidityExpire(30000);

Signed-off-by: Kiril Petrov <vscp@geomi.org>