Added alarm pulsing option to PVWidgetEditpart.

At DLS we use the red colour to indicate OFF, as well as to indicate an alarm state. This is confusing, so we have a requirement to make alarms pulse. This also makes the panels more friendly for colour blind people.

To implement this, a new boolean property "alarm_pulsing" (with default value false) has been added to the IPVWidgetModel, and an implementation in PVWidgetEditpartDelegate pulses the alarm colour of any alarm sensitive part of the widget around the midpoint of the normal colour and the alarm colour. Period is 1.5s for MAJOR, 3s for MINOR, and no pulse for DISCONNECTED.