DropTarget hint and DropHighlightAdorner background brush

[mangoyoga80]

What changed?

• Added some null checking in DefaultDropHandler.
• Minor changes to base class DropTargetAdorner with null check and some base properties

Feature DropTargetHighlightBrush

• Added new feature for changing the brush of highlight adorner.
• Feature is enabled through attached property dd:DragDrop.DropTargetHighlightBrush.
• Updated TreeView\Bound sample

Feature DropHint

• Added new feature for drop hint adorners to illustrate to the user what drop targets are available for a given drag source.
• Added attached property dd:DragDrop.UseDropTargetHint to enable the feature
• Added attached property dd:DragDrop.DropHintDataTemplate to override datatemplate for displaying hint
• Made DragDrop.TryGetDropHandler and DragDrop.TryGetDropInfoBuilder internal
• New method IDropTarget.DropHint and new properties on IDropInfo which may be breaking change
• dd:DragDrop.UseDropTargetHint added in SettingsView to enable for all demos using these settings
• The gif demonstrates the functionality
• Updated sample Mixed\DragDropContext to use Hint and Error hints

Closed issues.

487 Added drop target hints

488 Added the ability to change the brush of hint adorner.

[bsundsbo]

@punker76 any chance you are able to take a look at this PR in the near future?

Happy Easter