New types and message constants for DND support.
New types and message constants for DND support.

Also includes an abstract method that must be implemented by each backend
to enabled DND for components or forms.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI