Cleanup of no longer used OS handles on zoom changes

[akoch-yatta]

When a zoom changes occurs on a monitor it is possible, that some handles (e.g. fonts created for a specific zoom level) are no longer required and should be cleaned up. Some initial ideas for this:

• How to identify which zoom levels are still valid and needed? aggregation of the zoom levels of all currently used shell
• When to do the cleanup? After each DPI change event
• How to track which ressources could be cleaned up.? There is already org.eclipse.swt.graphicsResourceTracker to identify resource leaks. Perhaps this can be extanded for that behaviour

[amartya4256]

Merged with eclipse-platform/eclipse.platform.swt#1477