This commit changes the type of
DPIZoomChangeRegistry::dpiZoomChangeHandlers from TreeMap to
ConcurrentSkipListMap as at times there can be classes which can
register their handlers amidst a DPI change event leading to concurrent
read and write which can lead to ConcurrentModificationException

contributes to https://github.com/eclipse-platform/eclipse.platform.swt/issues/62 and https://github.com/eclipse-platform/eclipse.platform.swt/issues/127