[FEATURE] Browser can now also trigger drag events

Needed markup <tag draggable="true" data-dnd-mimetype="text/plain" data-dnd-data="Hello World!"/>
Bug: At least in SWT the TextTransfer accepts the input but always provides null as the data.
Until fixed you should add a DNDListener and save the data also to the LocalSelectionTransfer and check it on the receiver side as well
[REFACTOR]
[INTERNAL] SWTException correctly handled
[REFACTOR,FIX]
[INTERNAL] DemoExplorer notifies about selections
[FEATURE] New busy wait method in ExecUtils
[FIX] In testing scenarios without guy it could happen that ExecUtils.busyWait never stops since no event trigger the event queue. Fixed
[FEATURE] ClipboardListener to check the Clipboard for content changes
[FEATURE] Extended the ClipboardListener by the functionality to detect flavour changed

This feature saves some computation time. Not only will observers only notified when necessary, but also hash computations take only place when at least one observer is interested in clipboard content changes (instead of flavour changes).
[FEATURE] New HandlerUtils allow the easy definition of custom paste handlers.

A custom paste action becomes activated if the widget in question get focused and the clipboard contains data of a specified mime type.
This way you can still make use of the default paste handler provide a different solution in specific situations.
[FEATURE, FIX] Finally the browser does offer full copy/paste support.

Normal paste actions are treated by the default paste handler. File and image paste actions are delegated to a custom paste handler that circumvents the problem that files get inserted by a string representing their paths. Images in Safari are pasted with a wekit-fake-url - a URL that is only valid in the running browser session.
The custom handler reads the corresponding images and includes a data-url based image.